Figure 1.

CD4 chimeras containing the β subunit intracellular loop cocluster with the AChR. A, Schematic showing construction of chimeric proteins, in which the CD4 extracellular and transmembrane domains were fused with the major intracellular (IC) loop of each AChR subunit. B, CD4-subunit loops were expressed transiently in Sol8 myotubes and then immunoprecipitated from cell extracts and immunoblotted with an antibody to the CD4 extracellular domain. All CD4-subunit loop chimeras were robustly expressed. C, Sol8 myotubes transfected with CD4-subunit loop chimeras were treated with agrin for 16 h, and then immunostained for surface CD4 and AChR. Only CD4-β loop coclustered with the AChR in response to agrin treatment (arrows). CD4 and all other CD4-subunit loops remained diffusely distributed on the myotube surface (CD4-γ and δ loops are not shown, but see Table 1). CD4-εCT denotes an additional chimera that includes the IC loop, fourth transmembrane domain and C terminus, which also failed to cluster. Scale bar, 20 μm.