Dynamic distribution of an antigen involved in differentiation of quail myoblasts transformed with Rous sarcoma virus: I. Requirement of its quantitative expression on the cell surface for myoblast fusion.

A monoclonal antibody, H-145, the antigen of which is a glycoprotein with a molecular weight of 116 kDa, inhibits the fusion of quail skeletal myoblasts transformed with the temperature-sensitive mutant of Rous sarcoma virus (ts-RSV) (QM-RSV cells). Its antigen shows a unique distribution pattern during myogenic differentiation, and is required continuously for the inhibition as reported previously. The H-145 antigen is expressed from as early as the presumptive myoblast stage, and its expression increases during differentiation. In presumptive myoblasts, H145 antigen is mainly accumulated in the Golgi apparatus. However, on transfer to conditions for differentiation, the antigen accumulated in the Golgi apparatus begins to become dispersed in the cytoplasm, and is gradually transported to the cell surface during differentiation, suggesting that transportation of H-145 antigen to the cell surface is required for myoblast fusion. To examine this possibility, we studied the effect of bafilomycin A1, which blocks transport of intracellular proteins between trans-Golgi cisternae and the cell surface. Treatment of QM-RSV cells with bafilomycin A1 inhibited myoblast fusion even at 41 degrees C, the temperature for myogenic differentiation. Under this condition, the antigen did not diffuse to the cell surface, but remained localized in the Golgi apparatus, as on culture at 35.5 degrees C, the non-differentiation condition. These results suggest that quantitative expression of H-145 antigen on the cell surface is a prerequisite for myoblast fusion upon differentiation of QM-RSV cells.