Inhibition of nebulin and connectin (titin) for assembly of actin filaments during myofibrillogenesis.

In order to examine the role of cytoskeletal scaffolding proteins, nebulin and connectin (titin), in actin dynamics during myofibrillogenesis, rhodamine (rh)-labeled actin was microinjected into cultured skeletal muscle cells in which the function of these proteins had been inhibited with their respective antibodies. In the nebulin function-inhibited cells, exogenously introduced actin formed irregularly distributed amorphous patches or bright foci inside the cells, but it was not incorporated into myofibrillar structures at any stage. Thus, the blockage of actin binding sites of nebulin seems to inhibit the association of actin monomers to the preexisting nebulin scaffold. In the cells inhibited with anti-connectin antibody, incorporation of rh-actin was similar to that in antibody-uninjected cells. These results support the idea that nebulin is related to the accessibility/exchangeability of actin into nascent myofibrils, but connectin does not have such a role in actin assembly. Since all antibodies recognizing different domains of nebulin filaments blocked actin incorporation along the entire length of actin filaments, inhibition of any domains of nebulin filaments seems to affect actin dynamics.