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Insulin plays a fundamental role in whole-body glucose homeostasis. Central to this is the hormone's ability to rapidly stimulate the rate of glucose transport into adipocytes and muscle cells [1]. Upon binding its receptor, insulin stimulates an intracellular signalling cascade that culminates in redistribution of glucose transporter proteins, specifically the GLUT4 isoform, from intracellular stores to the plasma membrane, a process termed ‘translocation’ [1,2]. This is an example of regulated membrane trafficking [3], a process that also underpins other aspects of physiology in a number of specialized cell types, for example neurotransmission in brain/neurons and release of hormone-containing vesicles from specialized secretory cells such as those found in pancreatic islets. These processes invoke a number of intriguing biological questions as follows. How is the machinery involved in these membrane trafficking events mobilized in response to a stimulus? How do the signalling pathways that detect the external stimulus interface with the trafficking machinery? Recent studies of insulin-stimulated GLUT4 translocation offer insight into such questions. In the present paper, we have reviewed these studies and draw parallels with other regulated trafficking systems.

Regulation and specificity of membrane trafficking are required to maintain organelle integrity while performing essential cellular transport. Membrane fusion events in all eukaryotic cells are facilitated by the formation of specific SNARE (soluble N-ethylmaleimide-sensitive fusion proteinattachment protein receptor) complexes between proteins on opposing lipid bilayers. Although regulation of SNARE complex assembly is not well understood, it is clear that two conserved protein families, the Sx (syntaxin) and the SM (Sec1p/Munc18) proteins, are central to this process. Sxs are a subfamily of SNARE proteins; in addition to the coiled-coil SNARE motif, Sxs possess an N-terminal, autonomously folded, triple-helical (Habc) domain. For some Sxs, it has been demonstrated that this Habc domain exerts an autoinhibitory effect on SNARE complex assembly by making intramolecular contacts with the SNARE motif. SM proteins regulate membrane fusion through interactions with their cognate Sxs. One hypothesis for SM protein function is that they facilitate a switch of the Sx from a closed to an open conformation, thus lifting the inhibitory action of the Habc domain and freeing the SNARE motif to participate in SNARE complexes. However, whether these regulatory mechanisms are conserved throughout the Sx/SM protein families remains contentious as it is not clear whether the closed conformation represents a universal feature of Sxs.

Intracellular membrane fusion steps in eukaryotes require the syntaxin family of SNARE (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor) proteins. Syntaxins are regulated at several levels through interactions with regulatory proteins, including the SM (Sec1p/Munc18) proteins. Key to understanding this regulation is the characterization of different SM–syntaxin binding interactions at the molecular level and in terms of their contribution to function in vivo. The most conserved SM–syntaxin binding mode is through interaction of the syntaxin's extreme N-terminal peptide with a hydrophobic pocket on the surface of the SM protein. Surprisingly, mutant versions of two different SM proteins abrogated for this binding display no discernable phenotypes in vivo. In this issue of the Biochemical Journal, Johnson et al. demonstrate that loss of the N-terminal binding interaction between the syntaxin UNC-64 and the SM protein UNC-18 severely impairs neuromuscular synaptic transmission in Caenorhabditis elegans, resulting in an unco-ordinated phenotype. In contrast, loss of a second mode of SM–syntaxin binding has no detectable effect. Collectively, these results suggest that, although different membrane trafficking steps are all regulated by SM–syntaxin interactions using similar binding modes, they are differentially regulated, highlighting the need for careful dissection of the binding modes.