Cross-linked long-pitch actin dimer forms stoichiometric complexes with gelsolin segment 1 and/or deoxyribonuclease I that nonproductively interact with myosin subfragment 1.

Actin dimer cross-linked along the long pitch of the F-actin helix by N-(4-azido)-2-nitrophenyl (ANP) was purified by gel filtration. Purified dimers were found to polymerize on increasing the ionic strength, although at reduced rate and extent in comparison with native actin. Purified actin dimer interacts with the actin-binding proteins (ABPs) deoxyribonuclease I (DNase I) and gelsolin segment-1 (G1) as analyzed by gel filtration and native gel electrophoresis. Complex formation of the actin dimer with these ABPs inhibits its ability to polymerize. The interaction with rabbit skeletal muscle myosin subfragment 1 (S1) was analyzed for polymerized actin dimer and dimer complexed with gelsolin segment 1 or DNase I by measurement of the actin-stimulated myosin S1-ATPase and gel filtration. The data obtained indicate binding of subfragment 1 to actin dimer, albeit with considerably lower affinity than to F-actin. Polymerized actin dimer was able to stimulate the S1-ATPase activity to about 50% of the level of native F-actin. In contrast, the actin dimer complexed to DNase I or gelsolin segment 1 or to both proteins was unable to significantly stimulate the S1-ATPase. Similarly, G1:dimer complex at 20 microM stimulated the rate of release of subfragment 1 bound nucleotide (mant-ADP) only 1.6-fold in comparison to about 9-fold by native F-actin at a concentration of 0.5 microM. Using rapid kinetic techniques, a dissociation constant of 2.4 x 10 (-6) M for subfragment 1 binding to G1:dimer was determined in comparison to 3 x 10 (-8) M for native F-actin under identical conditions. Since the rate of association of subfragment 1 to G1:dimer was considerably lower than to native F-actin, we suspect that the ATP-hydrolysis by S1 was catalyzed before its association to the dimer. These data suggest an altered, nonproductive mode for the interaction of subfragment 1 with the isolated long-pitch actin dimer.