Sec22b-dependent assembly of endoplasmic reticulum Q-SNARE proteins

Abstract

SNARE (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor) proteins involved in membrane fusion usually contain a conserved alpha-helix (SNARE motif) that is flanked by a C-terminal transmembrane domain. They can be classified into Q-SNARE and R-SNARE based on the structural property of their motifs. Assembly of four SNARE motifs (Qa, b, c and R) is supposed to trigger membrane fusion. We have previously shown that ER (endoplasmic reticulum)-localized syntaxin 18 (Qa) forms a complex with BNIP1 (Qb), p31/Use1 (Qc), Sec22b (R) and several peripheral membrane proteins. In the present study, we examined the interaction of syntaxin 18 with other SNAREs using pulldown assays and CD spectroscopy. We found that the association of syntaxin 18 with Sec22b induces an increase in alpha-helicity of their SNARE motifs, which results in the formation of high-affinity binding sites for BNIP1 and p31. This R-SNARE-dependent Q-SNARE assembly is quite different from the assembly mechanisms of SNAREs localized in organelles other than the ER. The implication of the mechanism of ER SNARE assembly is discussed in the context of the physiological roles of the syntaxin 18 complex.

p31 Deficiency Influences Endoplasmic Reticulum Tubular Morphology and Cell Survival

“…We showed that p31 deficiency caused no effects on the interaction between syntaxin 18 and BNIP1 in cells (Fig. 3B) and in an in vitro system (3). Taken together, these results show that another component(s) in the syntaxin 18 complex might be involved in the homotypic fusion of the ER and that p31 seems to be a negative regulator of homotypic fusion of the ER because p31 deficiency caused enlargement of the ER membrane following vesiculation of the ER.…”

Loss of Soluble N-Ethylmaleimide-sensitive Factor Attachment Protein α (αSNAP) Induces Epithelial Cell Apoptosis via Down-regulation of Bcl-2 Expression and Disruption of the Golgi

“…Although our study provides the first evidence that ␣SNAP is essential for the Golgi integrity in living cells, it is consistent with several published reports implicating ␣SNAP in the regulation of the Golgi and ER homeostasis. Indeed, ␣SNAP was previously identified as a molecular constituent of two major SNARE complexes controlling the bidirectional vesicle trafficking between the Golgi and the ER (41)(42)(43)(44). One complex involving syntaxin-5 mediates the direct (anterograde) ER to Golgi transport, whereas the other complex involving syntaxin-18 regulates the opposite retrograde vesicle trafficking from the Golgi to the ER (67).…”

“…Impaired ER to Golgi Trafficking-Although ␣SNAP can regulate vesicle fusion with different cellular membranes, the majority of its binding partners are located at the Golgi and the ER (30,(41)(42)(43)(44). On the other hand, ER and Golgi are known generators of proapoptotic signaling because pharmacological disruption of these compartments results in profound cell death (10,11).…”

Organization of SNAREs within the Golgi Stack

“…A central acidic domain binds d-and a-subunits of the COPI coat that function in analogy to the clathrin adaptor and cage components, respectively (Andag et al 2001;Reilly et al 2001). The Dsl1 complex stabilizes a SNARE complex containing Ufe1p/syntaxin 18, Sec20p/BNIP1, and Use1p/p31 in the ER (Kraynack et al 2005;Aoki et al 2008). Interacting Rab proteins have not been identified.…”