The βm Protein, a Member of the X,K-ATPase β-Subunits Family, Is Located Intracellularly in Pig Skeletal Muscle

The sequence of the pig cDNA encoding the muscle-specific βm-protein, a member of the X,K-ATPase β-subunits family, was determined. Two alternatively spliced transcripts encoding polypeptide chains of 355 and 351 residues were identified. The tissue specificity of expression of βm and other X,K-ATPase β-subunit genes was studied by RT-PCR performed on 24 tissues from newborn pigs. The βm expression was shown to be highly tissue-specific, being detected at the highest level in skeletal muscle, at a lower level in heart, and at much lower level in skin. The βm transcripts are more abundant in the tissues from the newborn than adult. Immunoblotting and deglycosylation shift assay indicated that skeletal muscle membranes of newborn pigs contain βm protein with an electrophoretic mobility and carbohydrate content very similar to that of human βm. Fractionation of membranes from both newborn and adult pig skeletal muscles by isopycnic centrifugation revealed that the majority of the βm protein is concentrated in the sarcoplasmic reticulum-containing fractions. This intracellular location is a unique property that distinguishes the βm protein from other members of the X,K-ATPase β-subunit family.