Regulator of vesicle trafficking involved in cytokinesis and root hair development, but not required for cell elongation. Binds the syntaxin KNOLLE. Interacts with SEC6. Localized with its membrane receptor. During cytokinesis, localizes to the cell plate from a very early stage of cell plate formation and until the cell plate reaches the parental cell wall. Expressed throughout the plant, both in mitotically active and quiescent cells. Enriched in dividing tissues. Belongs to the STXBP/unc-18/SEC1 family.