Mediates endoplasmic reticulum to Golgi transport (PubMed:10930451, PubMed:14742712, PubMed:9506515). 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 (PubMed:18167358). Part of a ternary complex containing STX5A, NSFL1C and VCP (PubMed:9506515). Part of a unique SNARE complex composed of the Golgi SNAREs GOSR1, GOSR2 and YKT6. This complex also includes VTI1A (By similarity). Interacts with COG4 (By similarity). Interacts with GM130/GOLGA2 (PubMed:18167358). Interacts (via IxM motif) with SEC24C and SEC24D; mediates STX5 packaging into COPII-coated vesicles (By similarity). Interacts with VLDLR; this interaction mediates VLDLR translocation from the endoplasmic reticulum to the plasma membrane (By similarity). Localizes throughout the Golgi apparatus, but most abundant in the cis-most cisternae. Expressed in the brain, heart, spleen, lung, liver, kidney and testis. Produced by alternative initiation at Met-55 of isoform 1. Belongs to the syntaxin family.