Since mechanisms to convert bound GDP directly into GTP are unknown, the inactive GTPases are induced to release bound GDP by the action of distinct regulatory proteins called guanine nucleotide exchange factors or GEFs.
Some GTPases also bind to accessory proteins called guanine nucleotide dissociation inhibitors or GDIs that stabilize the inactive, GDP-bound state.
The following classification is based on shared features; some examples have mutations in the base-recognition motif that shift their substrate specificity, most commonly to ATP.
[8] Multiple classical translation factor family GTPases play important roles in initiation, elongation and termination of protein biosynthesis.
Sharing a similar mode of ribosome binding due to the β-EI domain following the GTPase, the most well-known members of the family are EF-1A/EF-Tu, EF-2/EF-G,[9] and class 2 release factors.
[2][3] The α and γ subunit are modified by lipid anchors to increase their association with the inner leaflet of the plasma membrane.
[2][3][15] In unstimulated cells, heterotrimeric G proteins are assembled as the GDP bound, inactive trimer (Gα-GDP-Gβγ complex).
Once Gα is returned to being GDP bound, the two parts of the heterotrimer re-associate to the original, inactive state.
[12] The Gβ and Gγ are likewise composed of many members, increasing heterotrimer structural and functional diversity.
[12] Among the target molecules of the specific G proteins are the second messenger-generating enzymes adenylyl cyclase and phospholipase C, as well as various ion channels.
[8] Named after the signal recognition particle (SRP), MinD, and BioD, the class is involved in protein localization, chromosome partitioning, and membrane transport.
[19] For a discussion of Translocation factors and the role of GTP, see signal recognition particle (SRP).