A tip-high gradient of a putative plasma membrane SNARE approximates the exocytotic gradient in hyphal apices of the fungus Neurospora crassa

Antibodies to the Saccharomyces cereviseae plasma membrane t-SNARE Sso2p id entify a putative 39-kDa homologue in Neurospora crassa. The 39-kDa protein is enriched in plasma membrane (PM) and occurred with other membranes. It is extractable by detergent, but not chaotropic or alkali agents, suggestin g membrane insertion. Immunoprecipitation with anti-Sso2p coprecipitated a similar to 100-kDa, MS+-ATP-sensitive band with the 39-kDa protein, suggest ing a ternary SNARE complex. Affinity-purified anti-Sso2p gave hyphal stain ing patterns most consistent with protein localization on both the PM and i ntracellular exocytotic apical wall vesicles. The PM staining in hyphal api ces formed a tip-high gradient, not as steep as that predicted by the "hyph oid equation," but closer to published gradients of cell wall matrix deposi tion. We conclude that the t-SNAREs are transported to the PM on the apical vesicles, but their tip-high gradient alone is insufficient to explain the vesicle fusion gradient in growing tips. (C) 2000 Academic Press.