Howard Hughes Medical Institute, Department of Developmental Biology, Beckman Center, Stanford University, Stanford, CA 94305, USA
(e-mail: rnusse{at}cmgm.stanford.edu)
SUMMARY
This review compares the signaling mechanisms of the Wnt and the Hedgehog proteins. Although Wnts and Hedgehogs are unrelated proteins, they are both modified by lipids, possibly through the action of enzymes that are related to each other. At the surface of target cells, the reception of Wnt and Hedgehog signals is regulated by several molecules, some of which, in particular the Frizzled and Smoothened receptors, are related to each other. Several other aspects of Wnt and Hedgehog transport and signaling are discussed, as well as the possible origin of these pathways.
Introduction
Animal development leads to numerous different outcomes, but the large
majority of signaling events in embryos involve only a handful of families of
molecules, including the Notch-Delta membrane molecules, the Fibroblast Growth
Factors (Fgfs), the Bone Morphogenetic Proteins (Bmps), the Hedgehogs (Hh) and
the Wnts. Surprisingly, evidence is emerging that indicates that Wnt and Hh
signaling are similar to each other in several respects, inviting speculation
that these signaling pathways are evolutionarily related. Among the features
they share are lipid-modified signals and the participation of cell-surface
receptors Frizzled (Fz) and Smoothened (Smo) that are related to each other.
In addition, both Wnt and Hh signaling use the protein kinases Gsk3 and
Ck1 to facilitate proteolysis of the key transcriptional effectors:
ß-catenin for Wnt and Cubitus interruptus (Ci) for Hh
(Kalderon, 2002
).
This review summarizes our current understanding of Wnt and Hh signaling,
focusing on the nature of the proteins, and the interactions between the
signals and cell surface molecules in these pathways. The lipid modification
of Wnt and Hh poses a number of interesting questions that are central to our
understanding of the developmental roles of these molecules. The biochemical
aspects of signaling will therefore be discussed in the context of the
developmental functions and the genetics of Wnt and Hh signaling. For
overviews of Wnt and Hh pathways within cells, an aspect that will not be
presented here, the reader is referred to various other reviews
(Bienz and Clevers, 2000;
Cadigan and Nusse, 1997
;
Ingham, 2001
;
Ingham and McMahon, 2001
;
Kalderon, 2000
;
Kalderon, 2002
;
McMahon, 2000
;
Moon et al., 2002
) and the Wnt
homepage (see
http://www.stanford.edu/~rnusse/wntwindow.html).
Wnt and Hh proteins: lipid-modified signals
Both the Wnt and the Hh proteins, which are not related in sequence to each
other, are destined for secretion. Because molecules that are secreted from
cells are commonly glycosylated but not, as far as known, acylated, the
discovery that Wnt and Hh proteins carry covalently attached palmitates
(Pepinsky et al., 1998;
Willert et al., 2003
) came as
a surprise (Fig. 1). In the
case of Hh, palmitoylation is one of two lipid modifications, the other one
being cholesterol (Porter et al.,
1996b
).
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Wnt genes and Wnt signaling events have been known for several decades
(reviewed by Cadigan and Nusse,
1997) but the nature of the active Wnt proteins themselves
remained unknown until recently. Persistent problems in solubilizing and
isolating active Wnt molecules hindered the biochemical characterization of
these proteins, and presented major problems to understanding how Wnt proteins
signal. The recent purification of Wnt proteins
(Willert et al., 2003
) has, to
some extent, now explained how Wnts might function by revealing that Wnt
molecules are palmitoylated and are therefore much more hydrophobic than was
previously predicted from their primary amino acid sequences
(Willert et al., 2003
)
(Fig. 1). The amino acid of Wnt
proteins that appears to be modified is the first conserved cysteine (C77), a
residue that is present in all Wnts and that is essential for Wnt function, as
revealed by mutant analysis (Willert et
al., 2003
). Treating Wnt with the enzyme acyl protein thioesterase
results in a form that is neither hydrophobic nor active, strengthening the
evidence that the palmitate is important for signaling. Wnt proteins are,
unlike Hedgehog proteins, usually glycosylated on conserved N-linked
glycosylation sites (Mason et al.,
1992
), although it is possible that they carry other modifications
as well, or that different forms of Wnt are palmitoylated at different
sites.
Wnts and Hedgehogs: similar enzymes for acylation?
Genetic observations had previously indicated that Wnts are lipid modified,
as genes with homology to acyltransferases are required for Wnt signaling.
These genes, porcupine (porc) in Drosophila
(Kadowaki et al., 1996) and
mom-1 in C. elegans
(Rocheleau et al., 1997
), are
required in Wnt-producing cells rather than for reception of the signal
(Fig. 1). In 2000, Hofmann
(Hofmann, 2000
) reported that
sequence similarities exist between Porc and membrane-bound acyltransferases,
enzymes that are present in the Endoplasmic Reticulum (ER) membrane and that
acylate a variety of substrates. Tanaka et al. have also observed that Porc
can bind to a domain in Wingless (Wg; the best characterized Wnt in
Drosophila) that encompasses the acylation site, providing more
evidence that Porc is the enzyme that is responsible for the acylation of Wnt
proteins, although this remains to be shown directly
(Tanaka et al., 2002
;
Willert et al., 2003
).
Strikingly, a gene with homology to porc is genetically required
for the production of active Hh. This gene was isolated by four different
groups and is therefore known by the names skinny hedgehog
(Chamoun et al., 2001),
sightless (Lee and Treisman,
2001
) central missing
(Amanai and Jiang, 2001
) and
rasp (Micchelli et al.,
2002
). (I use the FlyBase nomenclature of rasp for this
gene.) Similar to porc, mosaic analysis indicates that rasp
is required in Hh-producing cells
(Micchelli et al., 2002
).
Cultured cells in which rasp function is blocked by RNA interference
secrete a form of Hh that lacks palmitate
(Chamoun et al., 2001
). It is
likely, therefore, that Rasp is indeed the enzyme that acylates Hh, but direct
biochemical evidence for this is lacking. It is not known where Rasp resides
in cells, but, by analogy to Porc, a likely location is the ER
(Kadowaki et al., 1996
).
A consensus sequence for acylation is unknown, not only for Porc and Rasp,
but also for the numerous cytoplasmic proteins that are lipid modified
(Dunphy and Linder, 1998). It
is nevertheless interesting to note that the cysteines that are lipid modified
in Wnt and Hh are followed by several basic residues in either protein
(Fig. 2). This indicates that
the enzymes encoded by porc and rasp use a positively
charged stretch of amino acids, which is conserved in the known Wnt and Hh
proteins (not shown), as a recognition site.
Function of the lipids
The phenotypic similarities between animals with mutations in wnt
and porc/mom-1 indicate that Porc and MOM-1 are enzymes that are
specific to Wnt signaling, and underscores the significance of the lipid as an
integral component of Wnt activity. In particular, none of the numerous
functions of wg in Drosophila are detectable in the absence
of porc (Kadowaki et al.,
1996), whereas absence of rasp does not have any
detectable consequences for wg
(Micchelli et al., 2002
).
Although these genetic data indicate that lipid modification is required
for Wnt and Hh protein activity, certain conditions can be created under which
the palmitate is not absolutely essential for their function. Overexpressing
Wg in the fly, under the control of a strong promoter, can partially
circumvent the need for Porc function
(Noordermeer et al., 1995).
Similarly, wnt mutant-gene constructs that lack the palmitoylation
site can, when overexpressed in cells, produce a weak signal
(Willert et al., 2003
). In
addition, non-palmitoylated Hh protein is active at high concentrations
(Pepinsky et al., 1998
). It is
possible, therefore, that the lipid serves to localize the proteins to
membranes and that its absence can be overcome by high protein concentrations.
Furthermore, the cysteine that is acylated on the Hh molecule can be
substituted with various hydrophobic amino acids
(Pepinsky et al., 2000
) with
some retention of activity, an observation that indicates that the lipid might
target Hh to the surface membrane of a cell, where Hh functions.
A site-directed mutation in one of the endogenous Hh genes in the mouse
(sonic hedgehog) showed that loss of cholesterol modification
attenuates the range of Hh activity or perhaps signaling activity itself
(Lewis et al., 2001). It was
shown earlier that overexpression of cholesterol-free Hh (HhN) can overcome
this defect (Beachy et al.,
1997
). Both observations could imply that cholesterol enhances
activity by membrane targeting, but it should be taken into account that these
Hh mutants may lack palmitate as well. Indeed, removing rasp in
Drosophila eliminates the effects of overexpressed cholesterol-free
HhN (Chamoun et al., 2001
). If
Rasp is the enzyme that palmitoylates Hh, this interesting observation
suggests that the HhN molecules, while free of cholesterol, are to some extent
still palmitoylated. This palmitoylated form is then the active species,
blocked in its activity by the absence of Rasp.
Transport and release of Wnt and Hh
Lipid attachment is a common modification of cytoplasmic proteins
(Dunphy and Linder, 1998) and
is important for the membrane targeting of intracellular signaling molecules,
but, as far as it is known, it is rare in proteins that operate outside cells.
Are Wnt and Hh molecules actually released from cells and, if so, how? Or do
they always tether to membranes? It has also been reported that Wnt and Hh
proteins can act on cells away from their source, as concentration-dependent,
long-range morphogenetic signals (Roelink
et al., 1995
; Zecca et al.,
1996
; Zeng et al.,
2001
). What role, if any, do lipids play in the long-range
transport of these proteins?
The release of Hh from cells requires a dedicated transport molecule: a
protein called Dispatched (Disp). Initially found in Drosophila
(Burke et al., 1999) but
functionally conserved in mammals (Caspary
et al., 2002
; Kawakami et al.,
2002
), Disp is a multiple-pass, transmembrane protein
(Fig. 1). In the absence of
Disp, Hh is not secreted from cells and is unable to signal to neighboring
cells. Interestingly, non-cholesterol modified HhN is not dependent on Disp;
it is secreted and is fully active, suggesting that the primary function of
Disp is to transport cholesterol-modified Hh
(Burke et al., 1999
). However,
as mentioned above, it should be kept in mind that HhN also lacks palmitate
(Pepinsky et al., 1998
); it
therefore remains possible that Disp is specifically needed for the release of
palmitoylated Hh from cells.
There is no evidence of a similar transporter for Wnt molecules, although a
gene identified in C. elegans, mom-3, is required in Wnt-producing
cells (Rocheleau et al.,
1997). This gene (also called mig-14) remains to be
characterized molecularly.
It is not clear how the palmitate influences Wnt and Hh transport from one
cell to another. Variants of Hh that lack the palmitoylation site are secreted
from cells, perhaps more efficiently than is wild-type Hh
(Chamoun et al., 2001;
Pepinsky et al., 1998
). The
same is true for Hh protein in the absence of rasp
(Chamoun et al., 2001
);
however, the nonpalmitoylated Hh protein is not functional.
With respect to secretion, the effect of disrupting palmitoylation of the
Wnt proteins is strikingly different to disrupting palmitoylation of Hh. A
wg allele (S21), in which the palmitoylated cysteine is mutated into
a tyrosine (Couso and Arias,
1994; Willert et al.,
2003
), results in a protein that is not secreted
(Fig. 3). The lack of secretion
of Wnt-mutant proteins is commonly seen and is usually attributed to protein
misfolding. More surprising is the observation that, in the absence of
porc, Wg is also retained by cells
(van den Heuvel et al., 1993
)
(Fig. 3). At first glance, it
might seem paradoxical that a Wnt without lipid is not secreted, in particular
in comparison with Hh, as one might expect that a less hydrophobic variant is
better released from cells. This difference is likely explained by the overall
structure of Wnt, which is a molecule that is rich in cysteines that are
presumably disulphide linked. In the absence of porc, the C77 residue
will have a free sulfhydryl group that may interfere with normal disulphide
formation of other cysteines, leading to a misfolded and retained protein
(Fig. 4). The palmitate on Hh
is also attached to a cysteine, but through an amide at the N terminus, which
leaves the sulfhydryl group free. Thus, the lack of palmitate on Hh (in the
rasp mutant) does not change the overall number of free sulfhydryl
groups (Fig. 4). In fact, none
of the three cysteines in Hh are involved in disulphide formation
(Hall et al., 1997
).
|
Proteoglycans in Wnt and Hh transport?
Heparin-sulfated forms of proteoglycans (HSPG) are long proteins with
branched sugar side chains that are expressed on the cell surface, often
through a GPI anchor (Nybakken and
Perrimon, 2002). HSPGs can form complexes with a variety of
signaling molecules, including Fgfs and Wnts
(Nybakken and Perrimon, 2002
).
Genetic evidence has implicated HSPGs in Wnt signaling, both in
Drosophila (Lin and Perrimon,
1999
; Tsuda et al.,
1999
), and in mouse mammary tumorigenesis
(Alexander et al., 2000
).
Absence of Dally, a gene encoding an HSPG in Drosophila,
generates phenotypes similar to wg
(Lin and Perrimon, 1999
;
Tsuda et al., 1999
), as do
mutations in genes that encode enzymes that modify HSPG
(Baeg et al., 2001
;
Lin and Perrimon, 2000
). One
school of thought suggests that HSPGs act as co-receptors on target cells
(Fig. 5). However, cultured
cells that lack Dally are still able to respond to Wg
(Lum et al., 2003
). Although
redundancy with other HSPGs in these cell culture experiments is not excluded,
it is possible that HSPGs act in other steps of Wg signaling, such as in the
transport of Wg, and that they are not required for reception.
|
Reception of Wnt and Hh signals
Cells employ multiple receptors to receive instructions from Wnt and Hh
signals, in complex and little understood configurations
(Fig. 5). Remarkably, the
Frizzled (Fz) receptors for Wnts (Bhanot et
al., 1996) are related to the Smoothened (Smo) protein that is
necessary for Hh signaling (Alcedo et al.,
1996
; van den Heuvel and
Ingham, 1996
). Both receptors have seven transmembrane domains and
a long N-terminal extension called a CRD (cysteine-rich domain). As a group,
these molecules are more closely related to each other than they are to the
other families of serpentine receptors, of which there are many.
Although Fz and Smo are related to each other, it should be stressed that
the actual mechanisms of activation of Fz and Smo are fundamentally different.
Smo is not thought to interact with an extracellular ligand, whereas Wnt
proteins bind directly to the CRD of Fz
(Bhanot et al., 1996;
Dann et al., 2001
;
Hsieh et al., 1999b
). In
vertebrates, little is known about the specificity that exists between Wnt
ligands and receptors, but, in Drosophila, affinity between
genetically matched pairs of Wnts and Fzs is high
(Rulifson et al., 2000
;
Wu and Nusse, 2002
).
The way in which Hh engages with its receptor has been subject to debate,
but a consensus has emerged in which Hh binds to Patched (Ptc)
(Chen and Struhl, 1996;
Ingham et al., 1991
;
Stone et al., 1996
), a 12-pass
transmembrane protein related to Disp. As a consequence of this interaction,
Smo is activated, or, to be more precise, is released from an inhibitory
activity that is exerted by Ptc when Ptc is not engaged by ligand. In other
words, the negative interaction between Ptc and Smo is relieved by binding of
Hh to Ptc, which turns Ptc activity off. Inhibition of Smo by Ptc may not by
direct binding, but rather by a catalytic activity of Ptc
(Taipale et al., 2002
). As Smo
can be both activated (Frank-Kamenetsky et
al., 2002
) and inhibited
(Taipale et al., 2000
) by
small molecules, it has been suggested that Ptc might act as the transporter
of an endogenous small compound that interacts with Smo
(Taipale et al., 2002
). Smo
can be activated to a constitutive, i.e. Ptc-independent, state by specific
point mutations found in tumors (Taipale
et al., 2000
; Xie et al.,
1998
), and it is thought that these mutant forms of Smo are
refractory to inhibition by small molecules
(Frank-Kamenetsky et al.,
2002
).
In the context of comparing Smo and Fz in this review, it may be useful to
include some additional observations on these molecules, including some
unpublished results from our own lab. First, there is no evidence that Smo can
bind a Wnt-like ligand. There are no known genetic interactions between Wnt
genes and smo, and, in direct binding assays, the Smo-CRD failed to
interact with any of the Drosophila Wnt proteins tested
(Wu and Nusse, 2002). Second,
the small molecules that modulate Smo activity, including cyclopamine
(Cooper et al., 1998
), do not
seem to have any effect on Wnt-Frizzled signaling in intact animals, in
particular in mice. This can be deduced from the phenotype generated by these
compounds in vivo (Frank-Kamenetsky et
al., 2002
). Tissues that are affected by the small molecule
inhibitors include those malformed by absence of Hh
(Cooper et al., 1998
;
Frank-Kamenetsky et al.,
2002
), but they do not resemble any Wnt or Frizzled mutant
phenotypes. Third, the mutations that activate Smo in various cancers include
a tryptophane to alanine mutation in the last transmembrane domain of Smo
(Xie et al., 1998
). Despite
the conservation of that residue in Fz, mutating it in the same way does not
active Fz to a ligand-independent state (M. Brink and R.N., unpublished). It
seems therefore that Smo and Fz, despite their kinship, operate in different
ways.
Other receptors and cell surface molecules
Adding to the complexity of Wnt and Hh reception is the existence of
several other membrane molecules and alternative receptors for these signaling
pathways (Fig. 5). Wnt
signaling requires not only a functional Fz, but also the presence of a long,
single-pass transmembrane molecule of the Lrp (LDL receptor-related protein)
family, which is encoded by arrow in Drosophila
(Wehrli et al., 2000) and by
Lrp5 or Lrp6 in vertebrates
(Pinson et al., 2000
;
Tamai et al., 2000
). It has
been proposed (Tamai et al.,
2000
), but not always confirmed
(Wu and Nusse, 2002
), that Wnt
molecules can also bind to LRP and form a trimeric complex with a Frizzled.
The cytoplasmic tail of Lrp may interact directly with Axin, one of the
downstream components in Wnt signaling
(Mao et al., 2001
;
Tolwinski et al., 2003
). The
specificity of Lrp5/Lrp6/Arrow in the Wnt pathway is further illustrated by
the identification, both in flies and in mice, of genes that are required as
ER chaperones for transport and folding for Lrps. The chaperone genes,
boca (Culi and Mann,
2003
) and Mesd (Hsieh
et al., 2003
) have phenotypes very similar to Wnt pathway
components (Fig. 5).
In mammals, but not in Drosophila, a protein called megalin may
control the endocytic uptake of the Hh protein by acting as a direct binding
partner (McCarthy et al.,
2002). Moreover, mouse embryos lacking megalin are
phenotypically similar to Hh mutants. Megalin is an LDL receptor-related
protein (it is also called Lrp2), and is therefore related to Lrp5 and Lrp6,
the Wnt interacting members of the family.
A recent discovery in Drosophila has presented compelling evidence
that a different type of receptor for Wnt might exist. The guidance of axons
in the CNS is regulated by a member of the Wnt family, DWnt5 (Wnt5
FlyBase). Drosophila embryos mutant for DWnt5 are similar to those
lacking the transmembrane tyrosine kinase Derailed. Indeed, the DWnt5 protein
can bind to the Derailed cell-external domain
(Yoshikawa et al., 2003). This
domain contains a Wnt Inhibitory Factor (Wif) domain, which has previously
been shown to interact with Wnt molecules
(Fig. 5)
(Hsieh et al., 1999a
). There
is specificity of interaction between Derailed and Wnts, as is shown by the
inability of the Wg protein to bind to Derailed
(Yoshikawa et al., 2003
). It
might also be relevant that DWnt5 does not bind to any of the
Drosophila Fz molecules (Wu and
Nusse, 2002
). How Derailed couples to the cytoplasmic components
of signaling is not clear; the kinase domain seems to be dispensable for its
function (Yoshikawa et al.,
2001
). It is quite intriguing to find a different mode of Wnt
signaling that operates in the migration and positioning of cellular
projections, rather than in the cell fate decisions that most Wnts control.
One can speculate that other Wnts, including vertebrate Wnt5, might employ a
similar mechanism, and there is indeed evidence for Wnts acting as axonal
guidance molecules in vertebrates (Hall et
al., 2000
).
Extracellular inhibitors of Wnt and Hh signaling and feedback loops
Most, if not all, signaling pathways are subject to negative regulation or
feedback control (Freeman,
2000), and Wnt and Hh signaling are no exceptions. In
Drosophila, expression of several Frizzleds is controlled by Wg
activity, in such a way that Wg signaling is either inhibited or facilitated
by different receptor levels. Dfz2 (Fz2 FlyBase), the highest affinity
receptor for Wg, is downregulated by Wg signaling and this leads in turn to
lowered levels of active Wg protein
(Cadigan et al., 1998
). Dfz3
(Fz3 FlyBase), yet another but lower affinity Wg receptor, is induced
by Wg signaling (Sato et al.,
1999
; Sivasankaran et al.,
2000
) although the consequences of this regulation seem to be
minor as Dfz3 mutants have a limited phenotype, if any
(Sato et al., 1999
;
Sivasankaran et al., 2000
). Hh
signaling leads to elevation of the expression of Ptc on the cell surface,
which results in sequestration of the Hh ligand
(Chen and Struhl, 1996
).
Effectively therefore, Hh and Wnt control their own distribution by
manipulating the expression of their receptors. The complexity of these
circuits of regulation is further illustrated by the upregulation of Smo by
Hh, which occurs at the protein level
(Denef et al., 2000
;
Zhu et al., 2003
).
In vertebrates, the Hh signal controls Ptc expression
(Goodrich et al., 1997), but
it also induces the expression of a cell-surface protein, hedgehog inhibitory
protein (HIP), which binds to and sequesters Hh
(Fig. 5)
(Chuang and McMahon, 1999
). No
Drosophila counterpart of HIP has been found and, in general,
vertebrates seem to have acquired a much more sophisticated set of secreted
Wnt or Hh molecules than have insects or worms
(Fig. 5). For example, Wnt
signals in vertebrates can be inhibited by no less than 5 different secreted
factors (Fig. 5). Wif
(Hsieh et al., 1999a
),
frizzled-related protein (Frp or Frzb)
(Moon et al., 1997
;
Rattner et al., 1997
) and
Cerberus (Piccolo et al.,
1999
) bind to Wnt molecules themselves. As an alternative way of
blocking Wnt signaling, dickkopf (Dkk)
(Glinka et al., 1998
) and the
Wnt-modulatorin-surface-ectoderm (Wise) protein
(Itasaki et al., 2003
)
modulate Wnt signaling by interacting with Lrp5/Lrp6/arrow. The mechanism by
which Dkk acts is unusual: it removes Lrp from the cell surface through the
endocytic pathway, and it does so by interacting at the same time with Lrp and
with Kremen, another cell surface molecule
(Mao et al., 2002
).
None of these Wnt inhibitors have been found in the fly genome. However,
Drosophila contains a gene that inhibits Wg and that is induced by
the Wg signal: it is called wingful
(Gerlitz and Basler, 2002) or
Notum (Giraldez et al.,
2002
), and it may encode a hydrolytic enzyme whose substrate is
unknown. Possible vertebrate homologs remain to be characterized.
What is the evolutionary origin of Wnt and Hh signaling?
Given the similarities between Wnt and Hh signaling, it is tempting to
speculate about the origin of these pathways and to examine whether they can
be traced back to a common ancestral pathway in evolution. C. elegans
contains 5 different Wnt genes and three ß-catenins. Likewise, the
primitive diploblast Hydra contains a bona-fide Wnt and a set of Wnt pathway
genes (Hobmayer et al., 2000).
Dictyostelium has no Wnt, but a ß-catenin-like gene (called
aardvark) (Grimson et al.,
2000
) is involved in morphogenesis, and there are clearly
recognizable homologs of ß-catenin in plants as well
(Amador et al., 2001
). It is
possible, therefore, that an ancient ß-cateninbased mechanism was present
prior to the evolution of animals. By adding Wnt and Frizzleds, ß-catenin
activity became subject to control from other cells, a quintessential aspect
of organized multicellular life.
Sequence analysis has not revealed Hh-like genes in C. elegans or
Hydra (although the genome of the latter organism is not yet completely
known). However, the structure of the Hh molecule itself is very similar to
that of zinc hydrolases and other enzymes, including bacterial ones
(Hall et al., 1995). This
similarly has fuelled speculation that the Hh signaling system is derived from
an ancient metabolic pathway (Taipale and
Beachy, 2001
).
There are further interesting parallels between bacterial processes and
signaling in animals. The inner bacterial membrane contains several
translocators with an overall 12-transmembrane topology. Some of these
translocators serve as efflux pumps to clear toxic drugs from the cell. These
pumps contain multiple subunits (Murakami
et al., 2002), including a 12-transmembrane molecule of the
resistance-nodulation-cell division (Rnd) family, a proton-driven molecular
transporter. The overall organization of the Rnd proteins is similar to that
of Ptc/Disp, and there is conservation of some essential amino acids between
these molecules (Taipale et al.,
2002
). As mentioned earlier, Ptc may control the translocation of
small molecules over membranes in mammalian cells
(Taipale et al., 2002
).
Another class of translocators in the bacterial membrane belongs to the ABC
transporter family, ATP driven translocators. These molecules also have 12
transmembrane domains, but their topology and sequence appears to be different
from the Rnd/Ptc family (Yakushi et al.,
2000).
However, the function of these ABC transporters is reminiscent of Disp:
they translocate lipid-modified proteins over the inner membrane into the
periplasmic space (Yakushi et al.,
2000). Moreover, many bacterial lipoproteins carry palmitate as
their lipid, covalently linked to a cysteine at the N terminus of the protein
(Yakushi et al., 2000
). Among
the functions of bacterial lipoproteins is cell-to-cell communication, as
exemplified by the Tgl protein in Myxococcus (Nudleman and Kaiser,
personal communication). Hence, a signaling system based on lipid-modified
proteins and specific membrane translocators is ancient, and may have been the
founder of the Wnt and Hh signaling systems. By exploring the similarities
between these pathways and how they operate in different organisms, we should
be able to make considerable inroads into understanding their roles in
development and disease.
ACKNOWLEDGMENTS
Wendy Ching kindly provided the data for Fig. 3. I thank Eric Nudleman and for discussions about bacterial lipoproteins. The work in my lab is supported by the Howard Hughes Medical Institute.
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