AbstractHigh molecular weight microtubule‐associated proteins MAP1A and MAP2 form thin projections from microtubule surfaces and have been implicated in crosslinking microtubules and other cytoskeletal components. We have purified native MAP1A from bovine brain and have studied its interaction with G‐ and F‐actin. Using a solid‐phase immunoassay we show that MAP1A binds in a dose‐dependent manner to both G‐actin and F‐actin. Addition of MAP1A to F‐actin causes gelation of F‐actin and SDS‐PAGE analysis shows that MAP1A co‐sediments with the gelled network, under conditions where F‐actin alone does not pellet. The low apparent viscosity of F‐actin is markedly increased in the presence of MAP1A, suggesting that MAP1A can crosslink F‐actin. Co‐incubation experiments indicate that MAP1A and MAP2 may bind to common or overlapping sites on the actin molecule. The widespread distribution of MAP1A and its interaction with microtubules, actin, and intermediate filaments suggests that it may constitute an important determinant of neuronal and non‐neuronal cellular mor
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