Mediates endoplasmic reticulum to Golgi transport. Together with p115/USO1 and GM130/GOLGA2, involved in vesicle tethering and fusion at the cis-Golgi membrane to maintain the stacked and inter-connected structure of the Golgi apparatus. (Microbial infection) Required for the efficient production of infectious virion during human cytomegalovirus infection. Mechanistically, participates in the formation of the cytoplasmic viral assembly compartment where tegument acquisition and envelopment occur. Part of a ternary complex containing STX5A, NSFL1C and VCP (By similarity). Identified in a unique SNARE complex composed of the Golgi SNAREs GOSR1, GOSR2, YKT6 and VTI1A (By similarity). Interacts with COG4 (PubMed:19536132). Interacts with GM130/GOLGA2 (By similarity). Interacts (via IxM motif) with SEC24C and SEC24D; mediates STX5 packaging into COPII-coated vesicles (PubMed:18843296). Interacts with VLDLR; this interaction mediates VLDLR translocation from the endoplasmic reticulum to the plasma membrane (PubMed:23701949). Localizes throughout the Golgi apparatus, but most abundant in the cis-most cisternae. (Microbial infection) By human cytomegalovirus infection. Produced by alternative initiation at Met-55 of isoform 1. Produced by alternative splicing and alternative initiation. Belongs to the syntaxin family.