From the Howard Hughes Medical Institute and Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0295
Small GTPases are monomeric guanine
nucleotide-binding proteins of 20-25 kDa molecular mass. They play
major roles in the regulation of growth, morphogenesis, cell motility,
axonal guidance, cytokinesis, and trafficking through the Golgi,
nucleus, and endosomes. The first small GTPase to be discovered was
Ras, and there are now many members of the Ras superfamily of GTPases.
These are grouped in five subfamilies (Ras, Rho, ADP-ribosylation
factors (ARF), Rab, and Ran), and each subfamily is featured in this
minireview series. When GDP is bound, the GTPases are inactive, and
activation occurs when GDP is released and GTP is bound. This exchange
is accomplished by proteins called either guanine nucleotide exchange factors (GEFs) or GDP dissociation stimulators (GDSs). The active GTP-bound GTPases interact with a variety of effector proteins to
produce their cellular effects. Their activity is time-limited by their
intrinsic GTPase activity which is stimulated by GTPase-activating proteins (GAPs). GDP dissociation inhibitors (GDIs) have also been
identified for some GTPases, e.g. Rho and ARF.
In the first minireview of the series, Anne B. Vojtek and Channing J. Der discuss cell signaling through Ras. This GTPase is important
because it is a key regulator of cell growth and is found in mutated
oncogenic forms in a large number of human tumors. When specific
residues in Ras are mutated it becomes constitutively active
(insensitive to GAP action) and causes cell transformation. The first
signaling pathway involving Ras to be discovered was the Raf/MEK/ERK
cascade of protein kinases that leads to the stimulation of certain
transcription factors. In their review, Vojtek and Der point to
additional Ras effectors that have been identified and point out that
other pathways besides Raf/MEK/ERK contribute to malignant
transformation. These other Ras effectors include p120GAP, which
associates with p190RhoGAP, RalGDS, which targets Ral and other
proteins, RIN1, which enhances the transforming ability of Bcr/Abl, and
phosphatidylinositol (PI) 3-kinase, which generates PIP3,
an activator of the protein kinases Akt/PKB and PDK1. All these
proteins have demonstrated or potential roles in the control of cell
growth, morphology, and apoptosis.
The second minireview of the series by Deborah J. G. Mackay and
Alan Hall examines the Rho subfamily of GTPases. These proteins come in
three major subtypes, namely Rho, Rac, and Cdc42, which control the
actin cytoskeleton in distinct ways. The authors discuss the role of
certain proteins as effectors of Rho GTPases in the reorganization of
the cytoskeleton. One of these is p160Rho kinase, which alters myosin
light chain phosphorylation, thus regulating myosin filament assembly
and F-actin bundling. Another is the enzyme (PI-4P 5-kinase) that
synthesizes the regulatory lipid PIP2. This lipid affects
many proteins including the egrin/radixin/moesin (ERM) family of
proteins, which are essential for Rho- and Rac-mediated actin changes.
Another major role for the Rho proteins is the regulation of gene
transcription, and Mackay and Hall discuss the various pathways (JNK
and p38 mitogen-activated protein kinase) and the transcription factors
(serum response factor, NF The third minireview of the series by Joel Moss and Martha Vaughan
explores the ARF subfamily. The first of these was discovered as a
factor required for the ADP-ribosylation of the The fourth minireview of the series by Frauke Schimmöller, Iris
Simon, and Suzanne R. Pfeffer describes the Rab GTPases, which number
at least 30. These play key roles in the secretory and endocytic
pathways and are located in distinct cellular compartments. Rabs
facilitate the formation of v-SNARE·t-SNARE complexes, which are
integral components of vesicle trafficking. It is proposed that Rabs
act by recruiting specific docking factors (Exocyst, Rabaptins) from
the cytosol to facilitate pairing of the SNAREs. In line with other
small GTPases, Rabs are active in the GTP form, and several Rab-binding
proteins (Rabphilin, Rabaptin 5) keep them in this form and thus
influence vesicle fusion. Pfeffer and associates also consider the
evidence that Rabs are required for transport vesicles to form.
Finally, they note some key issues that need to be addressed, including
the problem of specificity, i.e. how Rabs facilitate vesicle
targeting to the appropriate membranes and how they induce the
formation of correct SNARE pairs.
The last minireview of the series is by Mary Shannon Moore and focuses
on Ran, a GTPase that plays a central role in protein and RNA
trafficking in and out of the nucleus. It is one of the most abundant
GTPases, and cells contain either one or a few isoforms. Macromolecules
travel in and out of the nucleus through nuclear pore complexes (NPCs)
and utilize different receptors and carriers. However, almost all the
receptors interact with GTP-Ran and are regulated by the Ran GTPase
cycle. One GEF for Ran is RCC1 (regulator of chromosomal condensation),
which is strategically placed inside the nucleus. A Ran GAP is also
described. Interestingly this protein is post-translationally modified
by a ubiquitin-like addition, which targets it appropriately to the
cytoplasmic entrance to the NPC. The NPC is a very complicated
structure with many proteins still uncharacterized. Important are the
repeat-containing nucleoporins, which form "tracks" by which
transport substrates pass through the NPC. Ran functions to trigger the
assembly or disassembly of transport complexes, and an important factor
is probably the difference in the concentration of GTP Ran between the
nucleus and cytoplasm, but there are many complexities to the process that are examined in the minireview.
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B) involved. Finally, they consider the
astonishingly large number of GEFs for Rho proteins and point out the
lack of information on their specific roles and regulation.
-subunit of the
heterotrimeric G protein Gs by cholera toxin. Subsequently it was found that ARFs comprised three classes and were critical components of several vesicular trafficking pathways. In the
minireview, the domain structures of the various GEFs for ARFs (ARNO,
cytohesin 1, and cytohesin 3) are analyzed in detail. All contain a
domain present in Sec7, a yeast gene involved in protein secretion.
This domain encodes the GEF activity, and the proteins also contain pleckstrin homology and other domains that bind PIP2 and
are responsible for membrane binding. The crystal structure of the ARNO
Sec7 domain is described in terms of the residues required for ARF
binding, and the sites on ARF1 involved in ARNO binding are also
analyzed. The properties of ARF GAPs are also discussed in terms of
their activation and membrane recruitment by lipids (diacylglycerol, PIP2) and receptors for KDEL proteins. In an afterword, the
authors point to the relative lack of information about the class II
and III ARFs and the molecules that operate and regulate vesicular trafficking.
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* This minireview will be reprinted in the 1998 Minireview Compendium, which will be available in December, 1998.