Na,K-ATPase in skeletal muscle: two populations of β-spectrin control localization in the sarcolemma but not partitioning between the sarcolemma and the transverse tubules

This is the authors' abstract. We don't add key points for this paper.

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.