Small GTPases (), also known as small G-proteins, are a family of
hydrolase enzymes
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...
that can
bind and
hydrolyze guanosine triphosphate (GTP). They are a type of
G-protein found in the
cytosol that are
homologous
Homology may refer to:
Sciences
Biology
*Homology (biology), any characteristic of biological organisms that is derived from a common ancestor
*Sequence homology, biological homology between DNA, RNA, or protein sequences
* Homologous chrom ...
to the alpha subunit of
heterotrimeric G-protein
Heterotrimeric G protein, also sometimes referred to as the ''"large" G proteins'' (as opposed to the subclass of smaller, monomeric small GTPases) are membrane-associated G proteins that form a heterotrimeric complex. The biggest non-structura ...
s, but unlike the alpha subunit of G proteins, a small GTPase can function independently as a hydrolase enzyme to bind to and hydrolyze a guanosine triphosphate (GTP) to form
guanosine diphosphate (GDP). The best-known members are the
Ras GTPases and hence they are sometimes called
Ras subfamily
Ras, from "Rat sarcoma virus", is a family of related proteins that are expressed in all animal cell lineages and organs. All Ras protein family members belong to a class of protein called small GTPase, and are involved in transmitting signals ...
GTPases.
A typical G-protein is active when bound to GTP and inactive when bound to GDP (i.e. when the GTP is hydrolyzed to GDP). The GDP can be then replaced by free GTP. Therefore, a G-protein can be switched on and off. GTP hydrolysis is accelerated by
GTPase activating proteins (GAPs), while GTP exchange is catalyzed by
guanine nucleotide exchange factors (GEFs). Activation of a GEF typically activates its cognate G-protein, while activation of a GAP results in inactivation of the cognate G-protein.
Guanosine nucleotide dissociation inhibitors
In molecular biology, the Guanosine dissociation inhibitors (GDIs) constitute a family of small GTPases that serve a regulatory role in vesicular membrane traffic. GDIs bind to the GDP-bound form of Rho and Rab small GTPases and not only prevent ...
(GDI) maintain small GTPases in the inactive state.
Small GTPases regulate a wide variety of processes in the
cell, including growth,
cellular differentiation
Cellular differentiation is the process in which a stem cell alters from one type to a differentiated one. Usually, the cell changes to a more specialized type. Differentiation happens multiple times during the development of a multicellular ...
, cell movement and
lipid vesicle transport.
The Ras superfamily
There are more than a hundred proteins in the Ras superfamily.
Based on structure, sequence and function, the Ras superfamily is divided into five main families, (Ras,
Rho
Rho (uppercase Ρ, lowercase ρ or ; el, ρο or el, ρω, label=none) is the 17th letter of the Greek alphabet. In the system of Greek numerals it has a value of 100. It is derived from Phoenician letter res . Its uppercase form uses the sa ...
,
Ran,
Rab and
Arf GTPases).
The Ras family itself is further divided into 6 subfamilies:
Ras,
Ral,
Rit,
Rap,
Rheb, and
Rad. ''Miro'' is a recent contributor to the superfamily.
Each subfamily shares the common core G domain, which provides essential GTPase and nucleotide exchange activity.
The surrounding sequence helps determine the functional specificity of the small GTPase, for example the 'Insert Loop', common to the Rho subfamily, specifically contributes to binding to effector proteins such as
IQGAP and
WASP.
The Ras family is generally responsible for cell proliferation, Rho for cell morphology, Ran for nuclear transport and Rab and Arf for vesicle transport.
See also
*
GTP-binding protein regulators
References
External links
*
{{Portal bar, Biology, border=no
EC 3.6.5
Peripheral membrane proteins