Immunochemical studies have suggested a tight association of syntaxin with N-type calcium channels. Syntaxin specifically interacts with the fusion proteins containing the cytoplasmic loop (L, H,) between homologous repeats II and III of the a~ subunit of the class B N-type calcium channel (alB) from rat brain, but not with those of the class A Q-type (alA) or the class S L-type (Q~ s) calcium channels. This interaction is mediated by an 87 amino acid sequence (773-859) containing two overlapping predicted helix-loop-helix domains. The 87 amino acid peptide can specifically block binding of native N-type calcium channels to syntaxin, indicating that this binding site is required for stable interaction of these two proteins. Interaction takes place with the C-terminal one-third of syntaxin (residues 181-288), which is thought to be anchored in the presynaptic plasma membrane. Our results suggest a direct interaction between the cytoplasmic domains of these two presynaptic membrane proteins that could have an important role in the targeting and docking of synaptic vesicles near N-type calcium channels, enabling tight structural and functional association of calcium entry sites and neurotransmitter release sites.