Vesicle trafficking protein that functions in the secretory pathway. Interacts either with VTI11 and SYP21, or with VTI11 and SYP22 in the prevacuolar compartment, or with VTI12 and SYP61 in the trans-Golgi network to form t-SNARE complexes. Expressed in root, leaf, stem, flower and silique. SYP51 and SYP52 may have redundant functions. Belongs to the syntaxin family.