Na,K-ATPase in skeletal muscle: two populations of beta-spectrin control localization in the sarcolemma but not partitioning between the sarcolemma and the transverse tubules.
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 alpha subunits of the Na,K-ATPase colocalize with beta-spectrin and ankyrin 3 in costameres, structures at the sarcolemma that lie over Z and M-lines and in longitudinal strands. In immunoprecipitates, the alpha1 and alpha2 subunits of the Na,K-ATPase as well as ankyrin 3 associate with beta-spectrin/alpha- fodrin heteromers and with a pool of beta-spectrin at the sarcolemma that does not contain alpha-fodrin. Myofibers of mutant mice lacking beta-spectrin (ja/ja) have a more uniform distribution of both the alpha1 and alpha2 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 beta-spectrin. The Na,K-ATPase and beta-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 alpha2 but not the alpha1 subunit of the Na,K-ATPase is present in transverse (t-) tubules. The alpha1 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 alpha1 subunit into the t-tubules.
Williams
M W
MW
Department of Physiology, University of Maryland School of Medicine, Baltimore, MD 21201, USA.
Resneck
W G
WG
Kaysser
T
T
Ursitti
J A
JA
Birkenmeier
C S
CS
Barker
J E
JE
Bloch
R J
RJ
D000818:Animals; D017487:Ankyrins / Q000378:metabolism; D015153:Blotting, Western; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D005455:Fluorescent Antibody Technique; D018482:Muscle, Skeletal / Q000201:enzymology*; D011233:Precipitin Tests; D011817:Rabbits; D051381:Rats; D017207:Rats, Sprague-Dawley; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D013049:Spectrin / Q000378:metabolism*; D013347:Subcellular Fractions / Q000201:enzymology
D017487:Ankyrins; D013049:Spectrin; D000254:Sodium-Potassium-Exchanging ATPase