DIFFERENT MODES OF INTERACTION OF 2 PEPTIDE-FRAGMENTS FROM SUBDOMAIN-4 OF RABBIT SKELETAL-MUSCLE ACTIN WITH ACTIN PROTOMERS

Previously, we reported that the 2.6 kDa peptide fragment extending fr om Arg-177 to Tyr-198 in rabbit skeletal muscle actin bound to actin i tself and inhibited its polymerization, while the 9.1 kDa peptide exte nding from Ser-199 to Tyr-279 in actin did not. The 2.6 kDa segment of actin was reported to contain one of the important actin-actin contac ts (Hori, K. and Morita, F. (1992) J. Biochem. 112, 401-408). In this paper, we show additional evidence that the rate of salt-induced incre ase in the fluorescence of pyrene-labeled actin was decreased in the p resence of the 2.6 kDa peptide. Conventional actin filaments were only scarcely observed in the presence of the 2.6 kDa peptide under an ele ctron microscope with a steady stare of fluorescence increase. Further more, the 2.6 kDa peptide was found to sever F-actin into short filame nt fragments. The 9.1 kDa peptide, on the other hand, neither inhibite d the fluorescence increment of pyrene-actin nor severed actin filamen ts. However, the 9.1 kDa peptide was found to increase the viscosity a nd fluorescence intensity of pyrene-G-actin and to form short actin fi laments in the absence of salts. Contact sites in the 9.1 kDa segment in actin may have a different mode of interaction with adjacent actin protomers in actin filaments from that of the 2.6 kDa segment.