ALTERNATE BINDING OF ACTIN AND CALMODULIN TO MULTIPLE SITES ON DYSTROPHIN

Mouse dystrophin protein sequence 1-385 and various deletion mutants w ere expressed in Escherichia coli as fusion proteins, and the binding of actin, calmodulin, and troponin C were characterized. The fusion pr otein-containing sequence 1-385 bound actin with an apparent dissociat ion constant of 129 +/- 65 nM as measured using a solid-phase immunoas say. High affinity was also observed with ultracentrifuge cosedimentat ion assays and biotinylated-actin binding assays. Results with deletio n mutants and analysis based upon sequence homology were consistent wi th two or three high affinity F-actin-binding sequences within this re gion of dystrophin at sequence positions 18-37 (ABS 1), 128-149 (ABS 2 ), and potentially at a new region called ABS 3 (86-120), A fusion pro tein lacking these sequences but containing dystrophin triple helix se quences also bound actin but with reduced affinity. Calmodulin binds t o dystrophin sequence 1-385 in a Ca2+-dependent manner and competitive ly inhibits F-actin binding. Results were consistent with two Ca2+ cal modulin-binding sites in this region of dystrophin at approximate sequ ence positions 18-42 (CBS 1) and 104-125 (CBS 2) with calmodulin affin ities of 2.1 +/- 1 and 1.6 +/- 1.2 mu M, respectively. Troponin C can substitute for calmodulin, although it binds with about 2-fold lower a ffinity. These results suggest that calmodulin (or troponin C) binding alternates with and may regulate F-actin binding by dystrophin much a s has been postulated for other cytoskeletal proteins which are homolo gous to dystrophin.