NATIVE TALIN IS A DUMBBELL-SHAPED HOMODIMER WHEN IT INTERACTS WITH ACTIN

Electron microscopy of glycerol-sprayed and rotary metal-shadowed tali n from human platelets reveals a dumbbell-shaped molecule with an aver age length of similar to 51 nm. Analytical ultracentrifugation of nati ve talin yields a single molecular species with an apparent molecular mass of 412 (+/-28.6) kDa and a sedimentation coefficient of s(20w) = 11.2. Chemical cross-linking with glutaraldehyde (GA) and correspondin g SDS-PAGE analysis show that the monomer band of talin can be quantit atively converted to a dimer band at GA concentrations greater than or equal to 0.45%, indicating that there is no significant amount of mon omer present in solution. These structural and biophysical data are co mpatible with native talin being an antiparallel homodimer. Length mea surements and viscometric and fluorescent assays of actin filaments po lymerized in the presence of native talin and of covalently cross-link ed talin dimers all yield similar effects: namely, increased nucleatio n and polymerization rates and an overall reduction of actin filament length. Hence, we conclude that talin in its native biological state i s a dimer when promoting nucleation of actin filaments. (C) 1993 Acade mic Press, Inc.