Vesicle trafficking protein that functions in the secretory pathway. Interacts with VTI11 and either SYP21, or SYP22, or SYP61 in the prevacuolar compartment, or with VTI12 and SYP61 in the trans-Golgi network to form t-SNARE complexes. A number of isoforms are produced. According to EST sequences. Expressed in root, leaf, stem, flower and silique. SYP51 and SYP52 may have redundant functions. The exact location of the SYP51/SYP61 complex is unclear, but is probably on the trans-Golgi network because of the likelihood of containing chiefly VTI12. Belongs to the syntaxin family.