Membrane proteins of the synaptic vesicle and the presynaptic plasma membrane together with soluble proteins form a secretory fusion complex conserved from yeast to neurons (Söllner, T., Whiteheart, S. W., Brunner, M., Erdjument-Bromage, H., Geromanos, S., Tempst, P., and Rothman, J. E. (1993) Nature 362, 318-324). Two of the membrane proteins have been localized in chromaffin cells, which secrete catecholamines stored in chromaffin granules. Syntaxin 1A and 1B are found in a plasma membrane-enriched fraction, whereas synaptobrevin is concentrated on the granules. Recombinant syntaxin 1A has been used in an affinity chromatography assay to isolate syntaxin receptor proteins of the chromaffin granules. Solubilized granule membranes contain a single protein with high affinity for syntaxin 1A. Sequencing revealed partial homology with Sec1p, a hydrophilic yeast protein acting late in the secretory process. Genetic suppressor analyses predicted the interaction of Sec1p with Sso1p, a yeast homologue of syntaxin 1A, and with Sec4p, a homologue of rab3A (Aalto, M., Ronne, H., and Keränen, 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 with respective sequences of a soluble protein, termed Munc-18, reported to be the only brain protein with affinity for recombinant syntaxin 1A (Hata, Y., Slaughter, C. A., and Südhof, T. C. (1993) Nature 366, 347-351). The mSec1/Munc-18 may be a receptor protein for syntaxin 1A on the transmitter vesicles mediating their interaction with the plasma membrane in docking and fusion.