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. Part of a ternary complex containing STX5A, NSFL1C and VCP (By similarity). 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 (By similarity). 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. Belongs to the syntaxin family.