Conserved structural features of the synaptic fusion complex: SNARE proteins reclassified as Q- and R-SNAREs

SNARE [soluble NSF (N-ethylmaleimide-sensitive fusion protein) attachment protein receptor] proteins are essential for membrane fusion and are conserved from yeast to humans. Sequence alignments of the most conserved regions were mapped onto the recently solved crystal structure of the heterotrimeric synaptic fusion complex. The association of the four α-helices in the synaptic fusion complex structure produces highly conserved layers of interacting amino acid side chains in the center of the four-helix bundle. Mutations in these layers reduce complex stability and cause defects in membrane traffic even in distantly related SNAREs. When syntaxin-4 is modeled into the synaptic fusion complex as a replacement of syntaxin-1A, no major steric clashes arise and the most variable amino acids localize to the outer surface of the complex. We conclude that the main structural features of the neuronal complex are highly conserved during evolution. On the basis of these features we have reclassified SNARE proteins into Q-SNAREs and R-SNAREs, and we propose that fusion-competent SNARE complexes generally consist of four-helix bundles composed of three Q-SNAREs and one R-SNARE.

SNARE蛋白（即可溶性NSF附着蛋白受体，soluble NSF (N-ethylmaleimide-sensitive fusion protein) attachment protein receptor）是介导膜融合的关键蛋白，在从酵母到人类的各类生物中均具有进化保守性。我们将该蛋白家族最保守区域的序列比对结果，映射至近期解析的异三聚体突触融合复合物的晶体结构之上。该突触融合复合物结构中的四条α螺旋相互结合，在四螺旋束的中心区域形成了高度保守的氨基酸侧链相互作用层。上述保守层上发生的突变会降低复合物的稳定性，即便在亲缘关系较远的SNARE蛋白中，也会引发膜运输功能缺陷。当以突触融合蛋白-4（syntaxin-4）替换突触融合蛋白-1A（syntaxin-1A）并建模至该突触融合复合物中时，未出现明显的空间位阻冲突，且变异度最高的氨基酸均定位于复合物的外表面。综上，我们认为神经元来源的突触融合复合物的核心结构特征在进化过程中高度保守。基于上述结构特征，我们将SNARE蛋白重新划分为Q型SNARE（Q-SNARE）与R型SNARE（R-SNARE）两类，并提出具备融合活性的SNARE复合物通常由包含3个Q型SNARE与1个R型SNARE的四螺旋束构成。