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 from Arg-177 to Tyr-198 in rabbit skeletal muscle actin bound to actin itself and inhibited its polymerization, while the 9.1 kDa peptide extending 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 contacts (Hori, K. and Morita, F. (1992) J. Biochem. 112, 401-408). In this paper, we show additional evidence that the rate of salt-induced increase in the fluorescence of pyrene-labeled actin was decreased in the presence of the 2.6 kDa peptide. Conventional actin filaments were only scarcely observed in the presence of the 2.6 kDa peptide under an electron microscope with a steady stare of fluorescence increase. Furthermore, the 2.6 kDa peptide was found to sever F-actin into short filament fragments. The 9.1 kDa peptide, on the other hand, neither inhibited the fluorescence increment of pyrene-actin nor severed actin filaments. However, the 9.1 kDa peptide was found to increase the viscosity and fluorescence intensity of pyrene-G-actin and to form short actin filaments 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.