Differences in the ionic interaction of actin with the motor domains of nonmuscle and muscle myosin II

Changes in the actin-myosin interface are thought to play an important role in microfilament-linked cellular movements. In this study, we compared the actin binding properties of the motor domain of Dictyostelium discoideum ( M765) and rabbit skeletal muscle myosin subfragment-1 (S1). The Dictyosteli um motor domain resembles S1(A2) (S1 carrying the A2 light chain) in its in teraction with G-actin. Similar to S1(A2), none of the Dictyostelium motor domain constructs induced G-actin polymerization. The affinity of monomeric actin (G-actin) was 20-fold lower far M765 than for S1(A2) but increasing the number of positive charges in the loop 2 region of the D. discoideum mo tor domain (residues 613-623) resulted in equivalent affinities of G-actin for M765 and for S1. Proteolytic cleavage and cross-linking approaches were used to show that M765, like S1, interacts via the loop 2 region with fila mentous actin (F-actin). For both types of myosin, F-actin prevents trypsin cleavage in the loop 2 region and F-actin segment 1-28 can be cross-linked to loop 2 residues by a carbodiimide-induced reaction. In contrast with th e S1, loop residues 559-565 of D. discoideum myosin was not cross-linked to F-actin, probably due to the lower number of positive charges. These resul ts confirm the importance of the loop 2 region of myosin for the interactio n with both G-actin and F-actin, regardless of the source of myosin. The di fferences observed in the way in which M765 and S1 interact with actin may be linked to more general differences in the structure of the actomyosin in terface of muscle and nonmuscle myosins.