GTP binds to Rab3A in a complex with Ca2+/calmodulin

Abstract

Ras-like small GTP-binding proteins of the Rab family regulate trafficking of the secretory or endocytic pathways. Rab3 proteins within the Rab family are expressed at high levels in neurons and endocrine cells, where they regulate release of dense-core granules and synaptic vesicles (SVs). Rab3A is present as either the soluble or the SV membrane-bound form in neurons that are dependent on the GDP- or GTP-bound states respectively. GDP dissociation inhibitor (GDI) is known to induce the dissociation of Rab3A from synaptic membranes when GTP is depleted. In an earlier study, Ca2+/calmodulin (CaM) was also shown to dissociate Rab3A from synaptic membranes by forming an equimolar complex with Rab3A in vitro. We have examined a possible role for Ca2+/CaM in modulating both the binding of guanine nucleotides to Rab3A and the GTPase activity of Rab3A. The basal level of Rab3A GTPase activity was not affected by an association with Ca2+/CaM. Ca2+/CaM—Rab3A complex that was formed in synaptic membranes was able to bind guanine nucleotides, whereas the Rab3A—GDI complex could not. In addition, Ca2+/CaM led to the replacement of the GDP molecule in the Rab3A—GDI complex with GTP in Rab3A. Taken together, these results suggest that CaM may have a role in stimulating GTP binding to Rab3A that is complexed with GDI, which leads to the formation of an active GTP-bound form of the Rab3A—Ca2+/CaM complex.

Rab3A and calmodulin regulate acrosomal exocytosis by mechanisms that do not require a direct interaction

“…The effect was not directly related to interaction with Rab3A, since it inhibited equally well exocytosis promoted by wild type Rab3A or by the mutant that does not bind calmodulin. Park et al [30] have recently reported that calmodulin may have a role in stimulating GTP binding to Rab3A. In our experiments, recombinant Rab3A was loaded with a non‐hydrolyzable GTP analog, bypassing the activation step.…”

Rab46 integrates Ca2+ and histamine signaling to regulate selective cargo release from Weibel-Palade bodies

“…The mechanisms by which ECs convert seemingly homogeneous Ca 2+ signals to an agonist-appropriate cellular response is unclear. Several studies have shown that calmodulin, when activated by Ca 2+ , plays a role in Rab activity (Coppola et al, 1999; Park et al, 2002; Zhu et al, 2016). However, the fundamental mechanism by which Rab GTPases distinguish an agonist-induced rise in intracellular Ca 2+ to elicit a context-dependent cellular response has remained elusive.…”

Immune evasion by Salmonella: exploiting the VPAC1/VIP axis in human monocytes

“…also reported a positive correlation between Ca 2+ concentration and VPAC1 density. Calcium binding by CAM1 forms Ca 2+ /CAM1 complexes that bind to Rab3a, which subsequently causes switching of Rab3a from a GDP‐bound (inactive) form to a GTP‐bound (active) form . It is possible that the increase in CAM1 and Rab3a mRNA that we show may increase calcium binding and active Rab3a and hence may increase VPAC1 binding and transport to the cell membrane.…”