Identification of functional domains in the cytoskeletal protein talin

The cytoskeletal protein talin potentially plays a key role in actin-membrane linkage. It is able to nucleate actin filament growth in vitro while binding simultaneously to lipid bilayers. Thrombin digestion of human platelet talin yields tow polypeptide domains of 200 kDa and 47 kDa. We have purified these fragments and analyzed their functional properties: the 200-kDa fragment was active in nucleating actin filament formation and reduced the viscosity of filamentous actin, comparable to the effects of the intact protein. The 47-kDa fragment was inactive in this respect. However, the 47-kDa polypeptide, but not the 200-kDa fragment, interacted specifically with large liposomes containing acidic phospholipids. This is demonstrated by selective, hydrophobic photolabeling of the 47-kDa fragment using phosphatidylserine liposomes containing trace amounts of a photoactivable phospholipid analogue and by selective co-sedimentation of this domain with the liposomes. The 200-kDa fragment, whether alone or in conjunction with the small fragment, neither incorporated significant amounts of label nor co-sedimented with the liposomes. We thus are able to attribute specialized functions to distinct domains on the talin molecule. These enable the protein to interact simultaneously with actin filaments and lipid membranes.