Rin, a Neuron-Specific and Calmodulin-Binding Small G-Protein, and Rit Define a Novel Subfamily of Ras Proteins

Abstract

cDNAs encoding two novel 25 kDa Ras-like proteins, Rit and Rin, were isolated from mouse retina using a degenerate PCR-based cloning strategy. Using the expressed sequence tag database, human orthologs were also obtained and sequenced. The protein sequences of Rit and Rin, which are 64% identical, are more similar to each other than to any known Ras protein. Their closest homologs in the databases are Mucor racemosus Ras2 and Ras3, to which they show approximately 48% identity. Rit and Rin both bind GTP in vitro. An unusual feature of their structure is that they lack a known recognition signal for C-terminal lipidation, a modification that is generally necessary for plasma membrane association among the Ras subfamily of proteins. Nonetheless, transiently expressed Rit and Rin are plasma membrane-localized. Both proteins contain a C-terminal cluster of basic amino acids, which could provide a mechanism for membrane association. Deletion analysis suggested that this region is important for Rit membrane binding but is not necessary for Rin. Rit, like most Ras-related proteins, is ubiquitously expressed. Rin, however, is unusual in that it is expressed only in neurons. In addition, Rin binds calmodulin through a C-terminal binding motif. These results suggest that Rit and Rin define a novel subfamily of Ras-related proteins, perhaps using a new mechanism of membrane association, and that Rin may be involved in calcium-mediated signaling within neurons.

Distinct ontogenic and regional expressions of newly identified Cajal-Retzius cell-specific genes during neocorticogenesis

“…The mutant of Cacna2d2 called ducky shows a reduction of a current density of VDCCs in Purkinje cells and impairs the dendritic formation of these cells (23). In addition, several intracellular calcium signaling molecules, including Dscr1l1, calretinin, Rit2, and Pcp4 (25)(26)(27), are selectively expressed in CR cells (Fig. 2 A).…”

Nerve Growth Factor-dependent Activation of the Small GTPase Rin

“…4 -8). The ability of Rin to interact directly with calmodulin in a Ca 2ϩ -dependent manner (16,18) suggest that intracellular calcium may play a role in the regulation of Rin activity. However, further studies will be needed to investigate the role of additional signaling pathways in the regulation of this small G protein.…”

“…These Ras-related proteins lack a known recognition signal for C-terminal lipidation, a modification that is generally necessary for the subcellular localization and biological function of the majority of Ras-like GTPases. Additional unique features of this Ras subfamily include a distinct but conserved G2 core effector domain and a C-terminal calmodulin-binding domain for Rin and RIC (16,18). As an initial step in the biological characterization of Rit and Rin, we investigated their ability to regulate signaling pathways used by other Ras family proteins to control cell growth and transformation.…”

Increasing complexity of Ras signaling

“…Using a PCR-based approach with degenerate primers corresponding to conserved regions in Ras and employing as template, mouse retinal cDNA, three novel small GTPases were cloned (Lee et al, 1996). One of the two GTPases, named Rin for Ras-like protein expressed in neurons, binds calmodulin in a Ca 2+ -dependent manner, suggesting it may be involved in mediating calcium-dependent signaling within neurons (Lee et al, 1996). Rit, on the other hand, stands for Ras-like protein expressed in many tissues.…”