Department of Cell and Developmental Biology, Oregon Health Sciences University, Portland, OR 97201-3098, USA
(e-mail: schnappb{at}ohsu.edu)
![]() |
Summary |
---|
Key words: Kinesin, Scaffold, Signal transduction, Motors, Cytoskeleton, Protein trafficking
![]() |
Introduction |
---|
For many years it was presumed there were `kinesin receptors', that is,
molecules whose sole purpose is to link kinesins to cargo. This presumption is
not supported by the recent work, which instead indicates that kinesin-cargo
linkers (Bowman et al., 2000;
Kamal et al., 2000
;
Lee et al., 2002
;
Nakagawa et al., 2000
;
Setou et al., 2000
;
Setou et al., 2002
;
Verhey et al., 2001
) are
`familiar faces' that have other functions. Most are adaptors or scaffolds.
Through their multiple protein binding sites, they organize molecular
assemblies that constitute the cargo and are themselves part of the cargo
(Fig. 1).
|
One of the first proteins to illustrate this new concept was the adaptor
protein AP-1. Through its interactions with clathrin and specific
receptor-ligand complexes (Pearse,
1988), AP-1 coordinates the formation of specific vesicle
populations at the Golgi apparatus and the plasma membrane (reviewed in
Kirchhausen, 2002
). The new
idea contributed by kinesin research
(Nakagawa et al., 2000
) is
that a site on AP-1 also interacts with the tail of at least one kinesin
KIF13A. This interaction explains how a subset of Golgi-derived
vesicles (those containing the mannose-6-phosphate receptor and its ligands)
is transported along microtubules to the pre-lysosomal compartment
(Nakagawa et al., 2000
). This
is an appealing idea: the same protein AP1 that sorts membrane
proteins into specific cargo vesicles also interacts with the motor that
transports these vesicles to their final destination.
The idea that kinesins are linked to adaptors that possess binding sites
for multiple proteins suggests we may have underestimated the role of
microtubule-based transport in cell biology. Given the large number of
scaffold proteins and kinesins encoded by the human genome, the range of
proteins that in principle could be part of one cargo or another is vast. As
the kinesin-cargo linker molecules and the proteins they scaffold come to
light, our notion of what constitutes `cargo' for intracellular transport
along microtubules will inevitably broaden. For example, recent studies
indicate that pre-assembled signal transduction cascades, or `transducisomes'
(Tsunoda et al., 1997), are
cargo for kinesins. Below, I discuss this novel role for kinesins at the
interface of signaling and transport.
![]() |
Trafficking of scaffolds for signaling pathways |
---|
Scaffold molecules are particularly important for the organization of PDZ-based signaling cascades. Many of these reside at specific subcellular sites in the nervous system, where the need for a high degree of localization, for example, at synaptic junctions, is obvious. Equally important is the need to localize signaling cascades in epithelia. Many intercellular signaling pathways that govern cell fate specification during embryonic development occur in the context of epithelia (Fig. 2). Like neurons, epithelial cells are asymmetric: the basolateral and apical domains encounter distinct extracellular environments. Consequently, communication from an inducing cell to a neighboring cell will depend on localization of the relevant signaling molecules.
|
The importance of trafficking and localizing signaling modules seems
obvious in these contexts and is supported by a great deal of circumstantial
evidence. However, only a few studies have actually provided direct evidence
that proper trafficking of signaling molecules to a highly localized
subcellular destination is essential for intercellular signaling and global
expression of signaling molecules is inadequate. Among the clearest cases is
the seminal study that identified a complex of scaffold proteins, known as the
LIN complex, in the context of vulva development in Caenorhabditis
elegans (Kaech et al.,
1998). In C. elegans, a canonical Ras MAP kinase
signaling pathway specifies an epithelial precursor cell to become a vulva
cell. This pathway is activated when an EGF receptor tyrosine kinase, LET27,
in the precursor cell binds EGF released by an `anchor cell'. Because the
anchor cell resides in the stroma (Fig.
2), the EGF it releases cannot penetrate the intercellular
junctions connecting the precursor cells. The products of the LIN2,
LIN7 and LIN10 genes prevent differentiation of the precursors
by interfering with their ability to establish a localized cluster of LET27 at
the basolateral aspect of the junctional complex. These three proteins form an
evolutionarily conserved PDZ-based scaffold complex that interacts with LET27,
as well as a number of different receptors in various organisms and cell types
(Bredt, 1998
). In worms, null
mutations in the LIN2, LIN7 or LIN10 genes lead to the same
phenotype: LET27 is still present in the precursor cells, and even exists in
the basolateral compartment, but it is diffusely distributed and not
concentrated at the cell junction. Second allele complementation experiments
elegantly prove that specific interactions between the LIN scaffold complex
and LET27 are required for proper localization of the receptor LET27 and
operation of the pathway (Kaech et al.,
1998
). These studies provide direct experimental evidence that
expression and global distribution of the relevant signaling molecules in a
cell are not sufficient for signaling to occur normally. These molecules need
to be properly trafficked and localized.
In summary, it is clear that signaling cascades must be trafficked and localized, and that these activities involve scaffold proteins. But how is the localization of signaling modules achieved? And exactly where in the cell, and by what mechanisms, do signaling molecules load onto scaffolds? Until recently, it was widely presumed that scaffold proteins and their signaling molecule partners arrive at their destinations, where they assemble locally, by diffusion. Recent research on kinesin-cargo interactions instead suggests that signaling molecules are loaded onto their scaffolds away from their final destinations and that active transport along microtubules delivers these pre-assembled signaling modules to particular destinations. I will review how these new ideas developed from efforts in the kinesin field to define the nature of the motor-cargo linkage and the identity of the cargoes carried by kinesins, discussing this recent work chronologically because of the compelling manner in which the experiments and ideas progressed.
![]() |
KIF 17 traffics glutamate receptors in the brain |
---|
In light of these previous studies, the discovery that LIN-10 interacts
with KIF17 immediately raised the possibility that postsynaptic clusters of
glutamate receptors are established, at least in part, through KIF17-driven
transport (Setou et al., 2000)
(Fig. 3). In this context, the
LIN complex serves as a kinesin-cargo linker: it connects KIF17 to the
transmembrane protein NR2B (a subunit of the NMDA-sensitive glutamate
receptor), which itself provides the connection to a vesicle (Figs
1 and
3). In vitro vesicle motility
experiments support this model (Setou et
al., 2000
): dominant inhibitory KIF17 constructs inhibit
nucleotide-dependent microtubule binding of NR2B-containing vesicles isolated
from the brain; and wild-type KIF17, but not mutants lacking the C-terminal
residues that bind LIN-10, promotes the motility of isolated NR2B-containing
vesicles along microtubules.
|
It is unclear whether the LIN complex also scaffolds signaling molecules
downstream of the receptors it carries, but other multi-PDZ domain proteins do
function in this manner (Harris and Lim,
2001; Sheng and Sala,
2001
). For example, InaD has five PDZ domains that each hold a
component of the phototransduction cascade in Drosophila
photoreceptors (Tsunoda et al.,
1997
). Generation of a normal light response depends on this
architecture. Mutations within single PDZ domains produce specific defects in
the light response, each associated with mislocalization, to the cytoplasm or
plasma membrane, of the PDZ partner. InaD thus localizes, and is responsible
for the targeting of, a highly organized transducisome
(Tsunoda et al., 1997
).
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Conventional kinesin (kinesin I) |
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Kinesin I structure, motility and regulation
Native kinesin I is a tetramer consisting of a kinesin heavy chain (KHC)
dimer and two kinesin light chains (KLCs)
(Fig. 4). The KLCs are
non-motor polypeptides that associate with the KHC dimer exclusively, that is,
they are not found at appreciable levels, apart from the KHC dimer, and, as a
general rule, the KHC dimer is always associated with KLCs
(Hackney et al., 1991). For
many years the functions of the KLCs were enigmatic, but we now know they
participate in two activities that presumably are coordinated: linking kinesin
I to cargo and regulating kinesin I motor activity.
|
As the vesicle-microtubule interface can only accommodate one or two motor
molecules, kinesin I (Block et al.,
1990; Howard et al.,
1989
) and other kinesins that carry small cargo
(Tomishige et al., 2002
) must
be processive, that is, they must walk along the microtubule without letting
go. If this were not the case, continuous transport of cargoes over long
distances would not be possible because the kinesin-cargo complex would
diffuse away from the microtubule. Although processivity would seem to imply
that the bulk of kinesin I should be bound tightly to microtubules, most
kinesin I is actually soluble, unbound to either microtubules or cargo.
Moreover, the microtubule-stimulated ATPase rate of isolated kinesin I is too
low (Hackney, 1995
) to account
for the
600 nm sec1 rate of kinesin-driven movement,
given that one ATP hydrolytic event powers each 8 nm step along the
microtubule (Schnitzer and Block,
1997
; Svoboda et al.,
1993
). What reconciles these apparent inconsistencies is that
kinesin I motor activity is turned off in the absence of cargo. If it were
not, the motor population would walk to the end of the tracks and accumulate
at the cell periphery. By negatively regulating motor activity, the cell
maintains the concentration of a processive motor such as kinesin I at a
uniform level throughout the cell and in large excess of cargo. An appealing
hypothesis is that a cargo controls its own destiny by binding to and
activating motors on demand (Verhey et
al., 1998
).
Kinesin I appears to be turned off in the absence of cargo by a
self-inhibition mechanism that depends on its tail and involves folding of the
KHC dimer (reviewed in Verhey and
Rapoport, 2001; Woehlke and
Schliwa, 2000
) (Figs
4 and
5). The KHC dimer contains
coiled coils interrupted by short unstructured regions that act as hinges
(Fig. 4). Isolated native
kinesin I is primarily in a folded conformation
(Fig. 5) that prevents the
motor domains from engaging the microtubule, and thereby inhibits motility and
ATP hydrolysis. The principal evidence supporting this model comes from
mutational analyses, which indicate that hinge 2
(Friedman and Vale, 1999
;
Seiler et al., 2000
) and
C-terminal residues of KHC (Hackney and
Stock, 2000
; Seiler et al.,
2000
; Stock et al.,
1999
; Verhey et al.,
1998
) are essential for inhibition. The C-terminal residues, which
carry excess positive charge, might interact with a region of excess negative
charge in the neck coiled coil (Stock et
al., 1999
) to stablilize the folded conformation.
|
Although such self-inhibition by folding is an integral property of the KHC
dimer, the KLCs are needed for full repression of KHC in vivo
(Verhey et al., 1998): in the
absence of KLCs, epitope-tagged KHCs colocalize with microtubules and
accumulate at the ends of cell processes, that is, at the minus ends of
microtubules. In vitro experiments also support the idea that the KLCs
contribute to inhibition (Friedman and
Vale, 1999
). Although the exact mechanism by which KLCs inhibit
kinesin I motor activity is unclear, the KLC structure-function relationships
provide some clues (Verhey et al.,
1998
). KLCs consist of at least two structurally distinct regions
(Fig. 4): an N-terminal region
of heptad repeats and a C-terminal region that has six tetratrico peptide
repeats (TPRs) protein-binding modules present in diverse proteins
(Blatch and Lassle, 1999
). The
heptad repeats alone are sufficient for binding KHC
(Fig. 4) and inhibiting the
ability of KHC to engage microtubules. By contrast, the KLC TPRs are required
for neither of these activities. In view of the idea that at some point KHC
motor activity must be activated in relation to cargo binding, it was
appealing to consider that the partner(s) of the KLC TPRs might be involved in
cargo binding or motor activation (Verhey
et al., 1998
). Thus, identifying the partners of the KLC TPRs
became a priority.
Kinesin I carries a MAP kinase cascade
We now know that the KLC TPRs are kinesin I cargo-binding domains.
Initially this came to light following the discovery that the TPRs interact
with a family of MAP kinase scaffold proteins the JIPs
(JNK-interacting proteins, known also as JSAPs)
(Bowman et al., 2000;
Verhey et al., 2001
). The
three JIP isoforms in mammals are scaffolds for the MAP kinase cascade that
activates Jun N-terminal kinase (JNK)
(Davis, 2000
). In common with
InaD and Ste5, JIPs juxtapose a cascade of kinases, ultimately enabling the
efficient and specific phosphorylation of JNK at two sites by a MAPKK
(Davis, 2000
). Activated JNK
phosphorylates downstream targets, including the transcription factor Jun, and
regulates various physiological activities
(Weston and Davis, 2002
),
including apoptosis in the brain
(Morishima et al., 2001
).
Among the many different transmembrane receptors implicated in JNK signaling
is ApoER2, the Reelin receptor (Stockinger
et al., 2000
). The Reelin signaling pathway has a very important
role in neurogenesis (Rice and Curran,
2001
), which is consistent with the enrichment of kinesin I and
JIPs in the brain.
The KLC TPRs recognize a motif found at the extreme C-termini of JIP 1 and
JIP2 (Fig. 3)
(Verhey et al., 2001). TPR
domains in other proteins likewise recognize the C-termini of their partners
(Gatto et al., 2000
;
Scheufler et al., 2000
;
Terlecky et al., 1995
). This
establishes a striking parallel between the kinesin-IJIP linkage and
the KIF17-LIN scaffold linkage (Fig.
3) with respect to how kinesin-cargo specificity is governed. Both
interactions involve binding modules, PDZ domains or TPRs, that recognize
motifs in the C-termini of their partners. In the case of kinesin I, the
module is in the kinesin and the motif in the linker, whereas in the case of
KIF17, the module is in the linker and the motif in the kinesin. But the
interactions are comparable fundamentally.
The significance of the JIP-KLC TPR interaction came to light in three
complementary studies (Bowman et al.,
2000; Byrd et al.,
2001
; Verhey et al.,
2001
). First, Bowman and co-workers identified Sunday Driver (SYD)
(Bowman et al., 2000
), the
Drosophila homologue of mammalian JIP3, in a mutant screen for genes
that produce in larvae a behavioral phenotype called tail flipping, which is
characteristic of kinesin I KHC and KLC null nutants
(Hurd and Saxton, 1996
;
Saxton et al., 1991
). This
phenotype is associated with axonal jams accumulations of membrane
organelles in peripheral nerves (Hurd and
Saxton, 1996
) and arises as the level of maternal kinesin
I decline during development, which leads to death. Although the exact
relationship between axonal jams and a specific kinesin I transport function
is unclear, the similar phenotypes of KLC, KHC and SYD mutants indicate that
the products of the three genes are involved in the same process. The
additional finding that SYD co-precipitates with kinesin I in vivo and
interacts directly with KLC TPRs suggested SYD functions by interacting
directly with kinesin I. But these studies left open the question of whether
SYD/JIP3 and its associated signaling molecules are kinesin I regulators,
cargoes or both.
Simultaneously, studies by Verhey et al.
(Verhey et al., 2001) provided
evidence that JIPs are cargoes of kinesin I in mammals. Earlier work had
indicated that JIPs are highly enriched at the tips of neuronal processes
(Kelkar et al., 2000
;
Meyer et al., 1999
), that is,
at the minus ends of microtubules, suggesting that they are transported there
by kinesin I. The fact that dominant inhibitory KLC constructs (heptad repeats
without TPRs or TPRs without heptad repeats) block neurite tip localization of
JIPs supports the hypothesis that JIPs are kinesin I cargo
(Verhey et al., 2001
). In
addition, the effects of mutations in JIP1 or JIP2 C-terminal residues on KLC
TPR binding and on neurite tip localization correlate
(Verhey et al., 2001
). Thus,
the steady-state distribution of JIP1 and JIP2 in neuronal cell lines depends
on the interaction between the conserved C-terminal residues of JIP1 and JIP2
and the KLC TPRs of kinesin I.
These studies generalize the model of kinesin-cargo architecture
established initially in the context of KIF17
(Fig. 3) and also contribute a
new idea: the JIP scaffold is pre-loaded with its kinase cascade prior to
transport. This idea was brought to light by the finding that not only JIP but
the kinases scaffolded by JIPs, and the transmembrane receptor ApoER2, all
co-precipitate from brain extracts with kinesin I when the latter is isolated
either with an antibody or by nucleotide-dependent microtubule
co-sedimentation (Verhey et al.,
2001). Furthermore, kinesin I dominant inhibitory constructs that
inhibit neurite tip localization of JIP likewise inhibit localization of the
MAPKK scaffolded by JIP (Verhey et al.,
2001
). These findings fit nicely with the notion of a
transducisome (Tsunoda et al.,
1997
) and raise the possibility that transducisomes are also
trafficking units. The findings also support the idea that signaling
scaffolds, in addition to juxtaposing kinases in a cascade, carry information
about the trafficking and localization of the cascade. This model differs from
the conventional view that signaling molecules assemble on scaffolds at their
final destination.
Does signaling through the cascade feedback to regulate the motor? One
appealing proposal (Verhey and Rapoport,
2001) is that activation of the Reelin pathway by ApoER2 causes
kinesin I to dissociate from the JIP scaffold upon fusion of the vesicle with
the nerve terminal membrane. Although this specific hypothesis has not been
tested, a third line of investigation
(Byrd et al., 2001
)
establishing an interaction between JIP scaffolding proteins and kinesin I
indicates that kinases scaffolded by JIP regulate intracellular vesicle
traffic, including cargo transport by kinesin I. These studies identified
unc-16, which encodes the JIP 3/SYD homologue, in a genetic screen
for molecules that organize presynaptic terminals in the C. elegans
nervous system. In C. elegans, as in mammals and flies, JIP 3
scaffolds JNK and its upstream kinases and is dependent on kinesin I for its
normal distribution in neurons (Byrd et
al., 2001
). Partial loss-of-function alleles of kinesin I,
unc-16 or JNK and its upstream kinases produce the same phenotype
mislocalization of synaptobrevin
(Byrd et al., 2001
)
and together enhance this phenotype; hence these genes must function in the
same process. Signaling via kinases carried by kinesin I thus does regulate
vesicle transport, but the nature of this regulation is unclear. Whether
kinesin I itself is regulated directly by the kinases it carries is yet to be
addressed.
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One kinesin multiple cargo-linkers |
---|
Definitive evidence now demonstrates that APP is transported in axons by
kinesin I (Gunawardena and Goldstein,
2001; Kamal et al.,
2000
). Earlier studies (reviewed in
Selkoe, 1998
) demonstrated
that APP is synthesized in the endoplasmic reticulum, glycosylated in the
Golgi apparatus, and packaged into vesicular structures that are transported
down axons (Yamazaki et al.,
1995
). Transport is blocked by anti-sense oligos against kinesin I
(Amaratunga et al., 1993
;
Ferreira et al., 1992
). The
new work establishes that the 47 C-terminal residues constituting the
cytoplasmic domain of APP interact directly with the KLC TPRs
(Kamal et al., 2000
) and that
kinesin I is associated with a defined class of axonally transported vesicles
that contain APP and the transmembrane proteins involved in its proteolytic
cleavage (Kamal et al., 2001
).
That the interaction between APP and KLC is required for APP transport is
clear from several experiments. Sciatic nerves from mice lacking one of the
three known KLC isoforms (KLC1, the neuronally enriched isoform) transport
less APP than nerves from normal animals
(Kamal et al., 2000
) and, in
Drosophila, deletion of the APP-like (appl) gene produces
the axonal jam phenotype in larvae that characterizes khc and
klc mutants (Gunawardena and
Goldstein, 2001
). Finally, the fraction of endogenous APP that has
been phosphorylated at Thr 668 by the neuronal kinase CDK5
(Iijima et al., 2000
)
accumulates at the tips of neurites in mammalian neuron-like cells in culture
(Ando et al., 1999
), and the
localization of this modified APP is abolished by overexpression of dominant
inhibitory KLC constructs (e.g. the TPRs or heptad repeats of KLC)
(Muresan et al., 2001
). This
finding, together with the observation that phosphorylation of APP Thr 668 by
CDK5 promotes the interaction between the APP C-terminus and KLC TPRs in vitro
(Muresan et al., 2001
),
supports the hypothesis that the transport of APP requires an interaction
between the cytoplasmic domain of APP and the KLC TPRs. The idea that APP is a
kinesin-I-cargo linker establishes a precedent for direct interactions between
kinesins and transmembrane proteins and thus suggests that the generalized
architecture in which kinesins are linked to transmembrane proteins indirectly
through soluble scaffolds (see Fig.
1), although common, is not universal.
Still unclear is whether APP and the JIP scaffold are carried on the same
cargo vesicles. Structural studies of the TPR motif indicate that three TPR
repeats would be sufficient to bind a partner
(Gatto et al., 2000;
Lapouge et al., 2000
;
Scheufler et al., 2000
); hence
it is conceivable that APP and JIPs bind simultaneously to the six TPR repeats
in a single KLC. Alternatively, a single kinesin I motor could carry JIPs and
APP simultaneously by devoting one KLC to each partner. A related question is
whether the JNK signaling cascade carried by kinesin I and APP are involved in
the same signaling pathway.
GRIP
The KLC TPRs do not provide the only cargo-binding site on kinesin I.
Earlier studies focusing on Neurospora kinesin I, which lacks KLCs,
identified a candidate cargo binding region on the KHC
(Fig. 4) (Seiler et al., 2000). Seiler
et al. searched for functional KHC domains, using KHC deletion mutant cDNAs
expressed as transgenes and evaluating their ability to rescue the reduced
growth rate of a KHC-deficient strain of Neurospora. They identified
in the tail coiled coil a domain of 51 residues
(Fig. 4) that is highly
conserved among KHCs from different species. In Neurospora, this
region is essential for localizing tagged KHC constructs to small vesicles
destined for secretion at the hyphal tip
(Seiler et al., 2000
). Setou
et al. subsequently used this region in a yeast two-hybrid screen of mouse
brain cDNAs (Setou et al.,
2002
) to identify the glutamate receptor interacting protein 1
(GRIP1), a known multi-PDZ-domain protein at synaptic junctions
(Dong et al., 1997
;
Srivastava et al., 1998
), as
the kinesin I partner. A region of GRIP1 between the sixth and seven PDZ
domains binds to a region of KHC that overlaps the cargo-binding domain
defined originally in Neurospora
(Fig. 4).
GRIP1, and the related protein GRIP2, scaffold several neuronal signaling
proteins (Wyszynski et al.,
2002). In particular, the fifth GRIP1 PDZ domain interacts with
the C-terminal sequence (-ESVKI) of subunits 2 and 3 (GluR2/3) of
AMPA-sensitive glutamate receptors
(Wyszynski et al., 1999
),
which mediate excitatory synaptic transmission in the brain. Indeed, GRIP was
initially identified through this interaction, and a large fraction of GluR2
exists as a complex with GRIP (Wyszynski
et al., 1999
). Although its exact function is unclear, GRIP
presumably contributes to the localization, at postsynaptic densities, of a
large multi-protein complex that transduces presynaptic activity into
postsynaptic responses.
GRIP1, GluR2 and kinesin I are colocalized within the dendrites and cell
bodies of cultured hippocampal neurons, and co-immunoprecipitate from vesicle
fractions (Setou et al.,
2002). That the interaction between kinesin I and GRIP1 is
functionally important for trafficking of AMPA-senstitive glutamate receptors
is evident from dominant inhibitory experiments: overexpression of the KHC
GRIP1-binding site reduces the amount of both GRIP1 and GluR2 in
dendrites.
The work described above provides further support for a generalized model
of kinesin-dependent trafficking of signaling modules that involves vesicles
and soluble scaffolds. It also raises interesting questions regarding the
logic of this trafficking. Using binding sites on KHC, kinesin I carries
AMPA-sensitive glutamate receptor vesicles within dendrites, but using binding
sites on KLCs it carries JIP or APP vesicles in axons. How does kinesin I,
which is distributed uniformly in both axons and dendrites, transport some
cargo vesicles to dendrites and others to axons? One interesting proposal is
that occupation of a particular cargo-binding site directs the motor to
distinct microtubules in axons and dendrites
(Setou et al., 2002).
![]() |
One cargo multiple kinesins? |
---|
In addition to their presence in non-motor proteins, MBS domains are found
in all members of the KIF1 subfamily of kinesins
(Asaba et al., 2003), next to
their FHA domains (Fig. 4). The
MBS domain was originally identified in GAKIN
(Hanada et al., 2000
)
the human homologue of KIF13b through its interaction with the protein
human disks large (hDlg) (Asaba et al.,
2003
). Dlg is a member of the membrane
associated guanylate-kinase MAGUK family of
scaffold proteins, which are composed of one or more PDZ domains, an SH3
domain and a guanylate kinase-like (GUK) domain that lacks enzymatic activity
and instead functions as a protein-binding module
(Anderson, 1996
). In humans,
the KIF13b MBS interacts with the GUK domain of Dlg
(Asaba et al., 2003
). This
interaction is extremely interesting because Dlg and other MAGUK proteins
localize signaling complexes at specialized membrane sites such as tight
junctions and synaptic junctions (Muller
et al., 1996
). For example, Drosophila Dlg and its
orthologues scaffold large protein complexes at pre- and postsynaptic
membranes and at the basolateral membrane of epithelial cells
(Lue et al., 1994
). The
scaffold protein LIN 2 (Fig. 2)
is also a MAGUK protein, and as discussed above, in C. elegans it
localizes an EGF receptor at tight junctions and glutamate receptors at
synaptic junctions. The finding that MBS domains exist in all KIF1 kinesins,
and the demonstration that the human KIF13b-MBSDlg interaction is
needed to maintain Dlg localization in MDCK cells
(Asaba et al., 2003
),
establishes a relationship between KIF1 kinesins and the MAGUKs. The finding
that the Liprin-
-binding site overlaps the MBS in KIF1A suggests that
the MBS also interacts with scaffold proteins that lack GUK domains. Thus, one
can imagine that the MBS of KIF1A, like the KLC TPRs of kinesin I, interacts
with more than one type of cargo linker.
The GRIPLiprin- interaction is clearly necessary for
localization of AMPA-sensitive glutamate receptors at postsynaptic sites on
the dendrites of cultured hippocampal neurons, as overexpressed GRIP or
Liprin-
constructs lacking the interaction sites diminish the
population of localized AMPA-sensitive glutamate receptors
(Wyszynski et al., 2002
). The
evidence that KIF1A transports glutamate receptors on the
Liprin-
GRIP scaffold is less direct. Liprin-
accumulates
in the cell bodies of hippocampal neurons that overexpress a KIF1A construct
lacking the motor domain (Lee et al.,
2002
). One presumes localization of AMPA-sensitive glutamate
receptors would be diminished, but this has not been shown directly.
Nevertheless, these studies again provide evidence for a signaling module
carried as vesicular cargo by a kinesin. Because GRIP and Liprin-
have
multiple binding sites for other proteins, this scaffold complex could in
principle hold components of a signaling pathway downstream of the glutamate
receptor or other receptors that interact with GRIP (reviewed in
Wyszynski et al., 2002
).
The implication that GRIP is carried by both KIF1A and kinesin I raises
several questions. Do KIF1A and Kinesin I mediate parallel, functionally
redundant transport pathways for the AMPA receptor? Or is AMPA receptor
delivery achieved in two consecutive transport steps might, for
example, one motor transport the cargo from the Golgi apparatus to the primary
dendrite, but the other mediate localized trafficking within the dendritic
tree? Assuming GRIP is carried by KIF1A within dendrites, how does this one,
ubiquitously distributed (Lee et al.,
2002), motor also deliver synaptic vesicle precursors within axons
(Hall and Hedgecock, 1991
;
Yonekawa et al., 1998
)?
![]() |
Summary and perspective |
---|
![]() |
Acknowledgments |
---|
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