LINKING MICROFILAMENTS TO INTRACELLULAR MEMBRANES - THE ACTIN-BINDINGAND VESICLE-ASSOCIATED PROTEIN COMITIN EXHIBITS A MANNOSE-SPECIFIC LECTIN ACTIVITY

Comitin is a 24 kDa actin-binding protein from Dictyostelium discoideu m that is located primarily on Golgi and vesicle membranes, We have pr obed the molecular basis of comitin's interaction with both actin and membranes using a series of truncation mutants obtained by expressing the appropriate cDNA in Escherichia coli, Comitin dimerizes in solutio n; its principle actin-binding activity is located between residues 90 and 135, The N-terminal 135 'core' residues of comitin contain a 3-fo ld sequence repeat that is homologous to several monocotyledon lectins and which retains key residues that determine these lectins' three-di mensional structure and mannose binding, These repeats of comitin appe ar to mediate its interaction with mannose residues in glycoproteins o r glycolipids on the cytoplasmic surface of membrane vesicles from D.d iscoideum, and comitin can be released from membranes with mannose, Ou r data indicate that comitin binds to vesicle membranes via mannose re sidues and, by way of its interaction with actin, links these membrane s to the cytoskeleton.