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* Department of Medicine, Division of Experimental Medicine, The emergence of processes from cells often
involves interactions between microtubules and microfilaments. Interactions between these two cytoskeletal 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 actin-binding protein-280
(ABP-280) and forms membrane blebs, which are not
seen in wild-type or ABP-transfected cells. The microinjection of tau or mature MAP2 rescued the blebbing phenotype; MAP2c not only caused cessation of
blebbing but also induced the formation of two distinct
cellular structures. These were actin-rich lamellae,
which often included membrane ruffles, and microtubule-bearing processes. The lamellae collapsed after treatment with cytochalasin D, and the processes retracted after treatment with colchicine. MAP2c was immunocytochemically visualized in zones of the cell that
were devoid of tubulin, such as regions within the
lamellae and in association with membrane ruffles. In
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 rheologic
property. These results suggest that MAP2c engages in
functionally specific interactions not only with microtubules but also with microfilaments.
Department of Neurology, Center for Neurologic Diseases,
Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts 02115
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