ABSTRACT
We used immunological approaches to study the factors controlling the distribution of the Na,K-ATPase in fast twitch skeletal muscle of the rat. Both α subunits of the Na,K-ATPase colocalize with β-spectrin and ankyrin 3 in costameres, structures at the sarcolemma that lie over Z and M-lines and in longitudinal strands. In immunoprecipitates, the α1 and α2 subunits of the Na,K-ATPase as well as ankyrin 3 associate with β-spectrin/α-fodrin heteromers and with a pool of β-spectrin at the sarcolemma that does not contain α-fodrin. Myofibers of mutant mice lacking β-spectrin (ja/ja) have a more uniform distribution of both the α1 and α2 subunits of the Na,K-ATPase in the sarcolemma, supporting the idea that the rectilinear sarcomeric pattern assumed by the Na,K-ATPase in wild-type muscle requires β-spectrin. The Na,K-ATPase and β-spectrin are distributed normally in muscle fibers of the nb/nb mouse, which lacks ankyrin 1, suggesting that this isoform of ankyrin is not necessary to link the Na,K-ATPase to the spectrin-based membrane skeleton. In immunofluorescence and subcellular fractionation experiments, the α2 but not the α1 subunit of the Na,K-ATPase is present in transverse (t-) tubules. The α1 subunit of the pump is not detected in increased amounts in the t-tubules of muscle from the ja/ja mouse, however. Our results suggest that the spectrin-based membrane skeleton, including ankyrin 3, concentrates both isoforms of the Na,K-ATPase in costameres, but that it does not play a significant role in restricting the entry of the α1 subunit into the t-tubules.
