Functional architecture of an intracellular membrane t-SNARE

Abstract

Lipid bilayer fusion is mediated by SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) located on the vesicle membrane (v-SNAREs) and the target membrane (t-SNAREs). The assembled v-SNARE/t-SNARE complex consists of a bundle of four helices, of which one is supplied by the v-SNARE and the other three by the t-SNARE. For t-SNAREs on the plasma membrane, the protein syntaxin supplies one helix and a SNAP-25 protein contributes the other two. Although there are numerous homologues of syntaxin on intracellular membranes, there are only two SNAP-25-related proteins in yeast, Sec9 and Spo20, both of which are localized to the plasma membrane and function in secretion and sporulation, respectively. What replaces SNAP-25 in t-SNAREs of intracellular membranes? Here we show that an intracellular t-SNARE is built from a 'heavy chain' homologous to syntaxin and two separate non-syntaxin 'light chains'. SNAP-25 may thus be the exception rather than the rule, having been derived from genes that encoded separate light chains that fused during evolution to produce a single gene encoding one protein with two helices.

Syntaxin 7, syntaxin 8, Vti1 and VAMP7 (vesicle-associated membrane protein 7) form an active SNARE complex for early macropinocytic compartment fusion in Dictyostelium discoideum

“…Consistent with the functional nomenclature defined in [34,36], two of these SNAREs (Vti1, Syn8) are co-t-SNAREs that are associated with Syn7 in a t-SNARE complex, whereas the third (VAMP7) is a v-SNARE of the VAMP\synaptobrevin family. The results highlight the remarkable conservation of functional as well as structural differences in co-t-SNAREs and v-SNAREs between Dictyostelium and other organisms.…”

“…These results set VAMP7 apart from Vti1 and Syn8. As expected for cot-SNAREs, Vti1 and Syn8 associate readily with Syn7, while VAMP7, acting as a v-SNARE, binds only to an already formed three-helix bundle [34,35].…”

The SNAP-25 Linker as an Adaptation Toward Fast Exocytosis

“…In most cases, the four SNARE-domains are encoded by separate membrane-targeted proteins, but the SNAREs driving the fusion of vesicles with the plasma membrane (exocytosis) are special in that three proteins provide the four domains (Weimbs et al, 1998;Fukuda et al, 2000). One of the SNAREs in this pathway, exemplified by the best-known isoform synaptosomal-associated protein of 25 kDa (SNAP-25), seems to have been created by fusion of the Qb and Qc SNAREs.…”

“…The SNARE domains can be subdivided into four homologous groups, named after their contribution to the zero layer: R, Qa, Qb, and Qc (Fasshauer et al, 1998b). SNARE assembly requires the participation of one domain from each group, resulting in a certain degree of specificity Scales et al, 2000a).In most cases, the four SNARE-domains are encoded by separate membrane-targeted proteins, but the SNAREs driving the fusion of vesicles with the plasma membrane (exocytosis) are special in that three proteins provide the four domains (Weimbs et al, 1998;Fukuda et al, 2000). One of the SNAREs in this pathway, exemplified by the best-known isoform synaptosomal-associated protein of 25 kDa (SNAP-25), seems to have been created by fusion of the Qb and Qc SNAREs.…”

Vsl1p cooperates with Fsv1p for vacuolar protein transport and homotypic fusion in Schizosaccharomyces pombe

“…Moreover, SNARE proteins are classified into four different groups (R-, Qa-, Qb-, and Qc-SNAREs), which form an extended four-helix bundle and are sufficient for complex formation. The combination of these three or four SNAREs is important for specificity of membrane transport Fukuda et al, 2000).…”