UTROPHIN ACTIN-BINDING DOMAIN - ANALYSIS OF ACTIN-BINDING AND CELLULAR TARGETING

Utrophin, or dystrophin-related protein, is an autosomal homologue of dystrophin. The protein is apparently ubiquitously expressed and in mu scle tissues the expression is developmentally regulated. Since utroph in has a similar domain structure to dystrophin it has been suggested that it could substitute for dystrophin in dystrophic muscle. Like dys trophin, utrophin has been shown to be associated with a membrane-boun d glycoprotein complex. Here we demonstrate that expressed regions of the predicted actin binding domain in the NH2 terminus of utrophin are able to bind to F-actin in vitro but do not interact with G-actin. Th e utrophin actin binding domain was also able to associate with actin- containing structures, stress fibres and focal contacts, when microinj ected into chick embryo fibroblasts. The expressed NH2-terminal 261 am ino acid domain of utrophin has an affinity for skeletal alpha-actin ( K-d 19+/-2.8 mu M), midway between that of the corresponding domains o f alpha-actinin (K-d 4 mu M) and dystrophin (K-d 44 mu M) Moreover, th is utrophin domain binds to non-muscle actin with a similar to 4-fold higher affinity than to skeletal muscle actin. These data (together wi th those of Matsumura et al. (1992) Nature, 360, 588-591) demonstrate for the first time that utrophin is capable of performing a functional ly equivalent role to that of dystrophin. The NH2 terminus of utrophin binds to actin and the COOH terminus binds to the membrane associated glycoprotein complex, thus in non-muscle and developing muscle utroph in performs the same predicted (spacer' or 'shock absorber' role as dy strophin in mature muscle tissues. These data suggest that utrophin co uld replace dystrophin functionally in dystrophic muscle.