'Artificial' myosin isozymes: preparation and characteristics.

Using precisely monitored proteolytic digestion conditions rabbit fast skeletal muscle myosin could be selectively modified in different ways. A myosin isozyme with a 20-kDa alkali light chain 1 (A1) could be obtained by digesting with papain in the presence of Ca2+. Under these conditions alkali light chain 2 (A2) was cleaved at Lys-17 and lost a 2.3-kDa N-terminal fragment including the strongly basic N terminus and about half of the characteristic (Ala-Pro) sequence. The Nbs2-[5,5' dithiobis(2-nitrobenzoic acid)-]light chain and A2 were left unmodified and less than 5% of the myosin heavy chain presented a break in the subfragment-2 region. EDTA and Ca2+ ATPase activities were unchanged. A myosin isozyme with an 18-kDa Nbs2-light chain was obtained by limited digestion with trypsin in the presence of Ca2+. The 18.9 leads to 18-kDa conversion was nearly 100% whereas less than 10% of the heavy chain was fragmented and only about 5% of A1 was converted to A1. The Nbs2-light chain was cleaved at Arg-7 preserving Ser-15 and consequently a phosphorylated modified myosin could be obtained. A quasi-elastic light-scattering study showed that this modified myosin in high-ionic-strength solutions exhibited physicochemical characteristics quite similar to those of unmodified myosin.