MICROTUBULE-ASSOCIATED PROTEIN 2C REORGANIZES BOTH MICROTUBULES AND MICROFILAMENTS INTO DISTINCT CYTOLOGICAL STRUCTURES IN AN ACTIN-BINDINGPROTEIN-280-DEFICIENT MELANOMA CELL-LINE

The emergence of processes from cells often involves interactions betw een microtubules and microfilaments, Interactions between these two cy toskeletal systems are particularly apparent in neuronal growth cones, The juvenile isoform of the neuronal microtubule-associated protein 2 (MAP2c) is present in growth cones, where we hypothesize it mediates interactions between microfilaments and microtubules. To approach this problem in vivo, we used the human melanoma cell, M2, which lacks act in-binding protein-280 (ABP-280) and forms membrane blebs, which are n ot seen in wild-type or ABP-transfected cells, The microinjection of t au or mature MAP2 rescued the blebbing phenotype; MAP2c not only cause d cessation of blebbing but also induced the formation of two distinct cellular structures, These were actin-rich lamellae, which often incl uded membrane ruffles, and microtubule-bearing processes, The lamellae collapsed after treatment with cytochalasin D, and the processes retr acted after treatment with colchicine. MAP2c was immunocytochemically visualized in zones of the cell that were devoid of tubulin, such as r egions within the lamellae and in association with membrane ruffles, I n vitro rheometry confirmed that MAP2c is an efficient actin gelation protein capable of organizing actin filaments into an isotropic array at very low concentrations; tau and mature MAP2 do not share this theo logic property. These results suggest that MAP2c engages in functional ly specific interactions not only with microtubules but also with micr ofilaments.