Organization of F-Actin by Avian Smooth Muscle Synaptopodin 2 (Fesselin)

Fesselin is a member of the synaptopodin family of actin binding proteins and it is rich in smooth muscle tissue. Fesselin accelerates actin polymerization in a calcium-calmodulin dependent fashion and forms insoluble aggregates with actin. The polymerization of actin occurs even in the absence of salts usually required to initiate polymerization. The question is whether the properties of fesselin are indicative of a function in forming structures rich in fesselin such as dense bodies and Z-lines. Electron microscopy of actin filaments shows that fesselin initially produces long actin filaments that become organized into thick bundles. The structure of the bundles depends on the ratio of fesselin to actin. Most frequently there are dense parallel ordered bundles. In some cases, the bundles are frayed and thus display single actin filaments. The filaments within a bundle are in-register and have either common initiation or termination points. Decoration with S1 showed that the filaments of a bundle had the same orientation. Actin filaments were occasionally seen to be radiating from densely stained complexes. Solution studies support the idea that fesselin stimulates pointed end growth of actin filaments. That is, fesselin (a) increased the critical concentration for polymerization and (b) induced actin growth even in the presence of barbed end blockers. When added to actin in the absence of other factors, fesselin can nucleate networks of linear actin filaments in which growth occurs primarily at the pointed ends of actin filaments.