Cap Z(36/32) is a contaminant and the major inhibitor of actin network formation in conventional actin preparations.

Previous studies have demonstrated that conventional actin preparations contain a potent factor which reduces the low shear viscosity of actin filaments. In this paper we demonstrate that Cap Z(36/32), a recently described protein from skeletal muscle that caps the barbed end of actin filaments and localizes to the Z-line of skeletal muscle, is the major factor affecting the low shear viscosity of actin prepared from muscle as described by Spudich and Watt.