Identification of the carboxyl-terminal membrane-anchoring region of HPC-1/syntaxin 1A with the substituted-cysteine-accessibility method and monoclonal antibodies.

HPC-1/syntaxin 1A is a member of the syntaxin family, and functions at the plasma membrane during membrane fusion as the target-soluble N-ethylmaleimide-sensitive factor-attachment protein receptor (t-SNARE). We identified the membrane-anchoring region of HPC-1/syntaxin 1A, and examined its role in anchoring of a protein to the plasma membrane. A series of mutants was created from a cysteine-less mutant of HPC-1/syntaxin 1A by substitution of each residue at the C-terminus with cysteine. The accessibility of the thiol-groups in each mutant was analyzed in vivo. The cysteine (C145) within the N-terminal cytosolic segment was labeled, but not that at C271 or C272, or any of those introduced at the C-terminus. The addition of additional residues to the C-terminal tail of HPC-1/syntaxin 1A allowed labeling by thiol-specific reagents. A monoclonal antibody directed against the C-terminal tail peptide did not react with the protein located at the plasma membrane. In addition, subcellular fractionation and immunocytochemical analyses with various transmembrane mutants showed that the C-terminal tail comprising eight amino acids is essential for anchoring of HPC-1/syntaxin 1A to the plasma membrane. These results indicate that the C-terminal membrane-anchoring region, which comprises 23 amino acids, does not traverse the lipid-bilayer and that the C-terminal tail is essential for anchoring of HPC-1/syntaxin 1A to the plasma membrane.