TARGETING, CAPTURE, AND STABILIZATION OF MICROTUBULES AT EARLY FOCAL ADHESIONS

By co-injecting fluorescent tubulin and vinculin into fish fibroblasts we have revealed a ''cross talk'' between microtubules and early site s of substrate contact. This mutuality was first indicated by the targ eting of vinculin-rich foci by microtubules during their growth toward s the cell periphery. In addition to passing directly over contact sit es, the ends of single microtubules could be observed to target severa l contacts in succession or the same contact repetitively, with interm ittent withdrawals. Targeting sometimes involved side-stepping, or the major re-routing of a microtubule, indicative of a guided, rather tha n a random process. The paths that microtubules followed into contacts were unrelated to the orientation of stress fiber assemblies and targ eting occurred also in mouse fibroblasts that lacked a system of inter mediate filaments. Further experiments with microtubule inhibitors sho wed that adhesion foci can: (a) capture microtubules and stabilize the m against disassembly by nocodazole; and (b), act as preferred sites o f microtubule polymerization, during either early recovery from nocoda zole, or brief treatment with taxol. From these and other findings we speculate that microtubules are guided into substrate contact sites an d through the motor-dependent delivery of signaling molecules serve to modulate their develop ment. It is further proposed this modulation p rovides the route whereby microtubules exert their influence on cell s hape and polarity.