Tropomyosin's Periods Are Quasi-Equivalent for Actin Binding but Have Specific Regulatory Functions

Abstract

Tropomyosin is a coiled coil that associates N-terminus to C-terminus to form a continuous strand along both sides of the actin filament and regulates its function. One long, high molecular weight tropomyosin molecule spans the length of seven actin subunits. In these forms there is a 7-fold periodicity in noninterface residues that have been proposed to correspond to seven quasi-equivalent actin binding sites. Interruption of the stable, canonical coiled coil by residues that destabilize the interhelical interface, such as Ala clusters, is required for actin binding. Previous studies have shown that the N-terminal half of period 5 (residues 165-188) is critical for actin binding and regulatory function and that both the surface "consensus" residues and the embedded, destabilizing Ala cluster are required for function. In the present work we test the hypothesis of quasi-equivalence of tropomyosin's periodic sites by replacing the proposed binding sites by substituting the crucial period 5 region with regions of period 1 or 2. Replacement mutants were designed to test the importance of the coincidence of the consensus residues and a destabilizing interface. The results show that generic (interface instability) and specific periodic surface residues are essential for function and that the periods tested (periods 1, 2, and 5) are quasi-equivalent for actin binding. However, regulatory functions are period-specific: periods 1 and 5 for binding to actin in the force-producing state and period 5 for Ca2+-dependent regulation with troponin.