IN CHROMAFFIN CELLS, THE MAMMALIAN SEC1P HOMOLOG IS A SYNTAXIN 1A-BINDING PROTEIN ASSOCIATED WITH CHROMAFFIN GRANULES

Membrane proteins of the synaptic vesicle and the presynaptic plasma m embrane together with soluble proteins form a secretory fusion complex conserved from yeast to neurons (Sollner, T., Whiteheart, S. W., Brun ner, M., Erdjument-Bromage, H., Geromanos, S., Tempst, P., and Rothman , J. E. (1993) Nature 362, 318- 324). Two of the membrane proteins hav e been localized in chromaffin cells, which secrete catecholamines sto red in chromaffin granules. Syntaxin 1A and 1B are found in a plasma m embrane-enriched fraction, whereas synaptobrevin is concentrated on th e granules. Recombinant syntaxin 1A has been used in an affinity chrom atography assay to isolate syntaxin receptor proteins of the chromaffi n granules. Solubilized granule membranes contain a single protein wit h high affinity for syntaxin 1A. Sequencing revealed partial homology with Sec1p, a hydrophilic yeast protein acting late in the secretory p rocess. Genetic suppressor analyses predicted the interaction of Sec1p with Sso1p, a yeast homologue of syntaxin 1A, and with Sec4p, a homol ogue of rab3A (Aalto, M., Ronne, H., and Keranen, S. (1993) EMBO J. 12 , 4095-4104). Although rab3A is present on chromaffin granules, we did not detect it bound to syntaxin 1A together with the mammalian Sec1p homologue (mSec1). The mSec1 peptide sequences are almost identical wi th respective sequences of a soluble protein, termed Munc-18, reported to be the only brain protein with affinity for recombinant syntaxin 1 A (Hata, Y., Slaughter, C. A, and Sudhof, T C. (1993) Nature 366, 347- 351). The mSec1/Munc-18 may be a receptor protein for syntaxin 1A on t he transmitter vesicles mediating their interaction with the plasma me mbrane in docking and fusion.