Characterization and Subcellular Distribution of G-Proteins in Highly Purified Skeletal Muscle Fractions from Rabbit and Frog

The presence of G-proteins in highly purified fractions from frog and rabbit skeletal muscles was analyzed by Bordetella pertussis toxin-catalyzed ADP-ribosylation and by immunoblots. Two B. pertussis toxin substrates were present in transverse tubules and sarcoplasmic reticulum from frog and rabbit skeletal muscle. Immunoblot analysis suggested that the two B. pertussis toxin substrates present in rabbit sarcoplasmic reticulum are possibly αi2 and αi3, but the corresponding substrates in sarcoplasmic reticulum from frog are still unidentified. Transverse tubules isolated from both rabbit and frog also contained αi2 and αi3; additionally, β35, β36, αo (weak immunoreactivity), αi1 (and/or αi3 in frog), as (short form only in frog), and αq were found. Sarcoplasmic reticulum from rabbit also contained αs (predominantly αs short), β36, and αq. The presence of αq in sarcoplasmic reticulum, in addition to transverse tubules, is potentially a very important finding since this G-protein has been described only in plasma membranes so far. Also, this study describes the candidate G-proteins for the control of excitation-contraction coupling in skeletal muscle.