HPC-1/syntaxin 1A (HPC-1), which has been identified as a presynaptic membrane protein, is believed to regulate the synaptic exocytosis as a component of t-SNARE. The distribution of the protein, however, is not restricted to the synaptic terminal, but it has been found to locate on the axonal membrane. When the expression of HPC-1 was suppressed, neurite sprouting was enhanced in cultured neurons. These findings suggest that HPC-1 possesses other functions than the regulation of the membrane fusion in neurotransmitter release. Rather it may also participate in the morphogenesis of neurons through membrane fusion, and possibly through cytoskeleton. HPC-1 has a sequence resemble to the assembly promoting sequence of heat stable MAPs in residues 89-106, suggesting that it can bind tubulin and be involved in microtubule system. Thus, both the tubulin binding property and the effect on microtubule assembly of HPC-1 were examined in vitro using a mutated HPC-1 lacking the C-terminal transmembrane region (HPC-deltaTM), which was overexpressed in E. coli. Affinity column chromatography showed that tubulin was found to bind HPC-1 directly. Synthetic peptide which corresponds to the residues 89-106 competitively inhibited the tubulin-HPC-1 binding, indicating that the sequence is responsible for the tubulin binding. In addition, chemical cross-linking with EDC revealed that one HPC-1 molecule can bind per one monomeric tubulin molecule. Light scattering measurement of microtubule polymerization showed that HPC-1 decreased the rate of the pure tubulin polymerization. Direct observation of single microtubules under dark-field microscopy showed that the growth rate of microtubule decreased by HPC-1. After shortening stopped, microtubules often spent attenuate phases, in which neither growing nor shortening was detected. When another mutant HPC-1 which is composed of residues 1-97 and lacks tubulin binding activity was used, however, the suppression of microtubule polymerization was not observed. These results suggest that HPC-1 is a potent regulator of microtubule polymerization, which directly bind tubulin subunit and decrease the polymerization activity.