These and other observations presented

These and other observations presented in the elegant, rigorous study by Thomas et al. (2018) clarify a puzzling enigma regarding the Rab specificities of yeast TRAPP complexes and further highlight an important though under-appreciated role for the C-terminal HVD of Rabs in GEF substrate selection. Steric gating in particular represents a conceptual shift wherein the length of the C-terminal HVD is exploited as a “molecular ruler” for specificity determination in conjunction with variations in amino Gossypol sequence. It will be interesting to elucidate the molecular and structural underpinnings and define the contribution of the HVDs to functional specificity more generally. Given numerous examples of catalytic/binding sites that recognize multiple unprenylated Rabs in vitro, a salient question motivated by this study is whether steric gating or analogous mechanisms have evolved to restrict the specificity of other GEFs, including mammalian TRAPP complexes, and, if GEFs, why not GAPs, effectors, or other binding partners and modifying enzymes? Working it out for the 60+ mammalian Rabs is far from trivial. Nevertheless, enquiring minds want to know!
Introduction Platelets are essential regulators of haemostasis, limiting blood loss following acute injury, while exacerbated platelet activity in diseased blood vessels is causative of ischaemic tissue damage due to occlusive thrombosis. These events are characterised by dynamic changes in platelet activation responses, including actin cytoskeleton remodelling (shape change and spreading), integrin activation (facilitating platelet aggregation), granule secretion (lysosomes, dense and α-granules), prostaglandin production (thromboxane A2) and phosphatidylserine exposure (facilitating thrombin-mediated coagulation) [1,2]. These functional responses are regulated by distinct signalling pathways propagated from various glycoprotein (GP) receptors (GPVI, GPIb-IX-V), G protein-coupled receptors (GPCRs – PAR1/4, TP, P2Y1/12) and integrin complexes (αIIbβ3, α2β1) which converge into common signalling pathways [3]. Among a range of identified signalling molecules important for platelet activation are the small GTPase proteins, in particular Rap1A and Rap1B (Ras family) and Rac1, RhoA, RhoG and Cdc42 (Rho family), which could represent suitable pharmacological targets for antithrombotic therapies [[4], [5], [6]]. Ral GTPases (RalA and RalB) are ubiquitously expressed members of the Ras subfamily of GTPases and are particularly abundant in the brain, testes and platelets [7]. Besides Rap1A and 1B, which have in excess of 100,000 copies/platelet, RalA and RalB are among the more highly expressed Ras GTPase members with 3400 and 6800 copies/platelet, respectively [8,9]. They target numerous effectors including Ral-binding protein 1 (RalBP1), phospholipase D and the exocyst complex to regulate cell polarity, exocytosis, autophagy and various cellular responses associated with tumorigenesis and metastasis [[10], [11], [12], [13], [14]]. In platelets, Rals have been previously shown to be activated in response to various agonists, dependent on rises in cytosolic Ca2+ [7]. A further study suggested a role for Rals in regulating platelet dense granule secretion in human platelets, which is consistent with reports in other cell types [15,16]. We recently showed however a restricted role for Rals in regulating exposure of P-selectin on the plasma membrane of mouse platelets [17]. Conditional deletion of either RalA or RalB in platelets showed no substantial alterations in P-selectin exposure, however deletion of both RalA and RalB (double knockout, DKO) in platelets produced a pronounced defect in this response. This confirmed a redundancy between Rals, which had been previously reported [13]. However, the defect was unusual in that release of soluble α-granule content, such as platelet factor 4 (PF4), was largely unaltered. This suggested a specific and novel role for Rals, specifically in the control of P-selectin exposure on the platelet surface.