IDENTIFICATION OF FUNCTIONAL DOMAINS IN THE CYTOSKELETAL PROTEIN TALIN

The cytoskeletal protein talin potentially plays a key role in actin-m embrane linkage. It is able to nucleate actin filament growth in vitro while binding simultaneously to lipid bilayers. Thrombin digestion of human platelet talin yields two polypeptide domains of 200 kDa and 47 kDa. We have purified these fragments and analyzed their functional p roperties: the 200-kDa fragment was active in nucleating actin filamen t formation and reduced the viscosity of filamentous actin, comparable to the effects of the intact protein. The 47-kDa fragment was inactiv e in this respect. However, the 47-kDa polypeptide, but not the 200-kD a fragment, interacted specifically with large liposomes containing ac idic phospholipids. This is demonstrated by selective, hydrophobic pho tolabeling of the 47-kDa fragment using phosphatidylserine liposomes c ontaining trace amounts of a photoactivatable phospholipid analogue an d by selective co-sedimentation of this domain with the liposomes. The 200-kDa fragment, whether alone or in conjunction with the small frag ment, neither incorporated significant amounts of label nor co-sedimen ted with the liposomes. We thus are able to attribute specialized func tions to distinct domains on the talin molecule. These enable the prot ein to interact simultaneously with actin filaments and lipid membrane s.