Binding of filamin isoforms to myofibrils

Two filamin isoforms were purified from bovine tissues and characterized. M uscle filamin and nonmuscle filamin had different SDS gel mobilities, prote olytic digestion patterns, myofibrillar binding distributions and myofibril binding affinities. The muscle specific filamin had an apparent molecular weight of 265 kDa and bound primarily to the Z-lines of myofibrils but also to the I-bands near the Z-lines. The nonmuscle specific filamin had an app arent molecular weight of 275 kDa and bound exclusively to the Z-lines of m yofibrils. The filamin-myofibril binding was studied quantitatively. Plotti ng bound fraction (mg filamin/mg myofibril) vs. equilibrium concentration o f free filamin yielded a biphasic binding curve. The first hyperbolic bindi ng phase described the binding of filamin to myofibrils but the second phas e appeared to be nonspecific due to filamin aggregation. The muscle filamin had a significantly lower (P < 0.05) apparent binding affinity to myofibri ls than nonmuscle filamin. However, the muscle filamin showed a significant ly higher (P < 0.05) saturation value for myofibrils than nonmuscle filamin . The binding of phosphorylated filamin to myofibrils was significantly low er (P < 0.05) than the corresponding native proteins for both filamin isofo rms.