Deduced amino acid sequence and E1–E2 equilibrium of the sarcoplasmic reticulum Ca2+‐ATPase of frog skeletal muscle Comparison with the Ca2+‐ATPase of rabbit fast twitch muscle

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Abstract
The cDNA encoding a Ca2+-transport ATPase of frog (Rana esculenia) skeletal muscle was isolated and characterized. The deduced amino acid sequence, consisting of 994 residues, showed 89% identity to the fast twitch muscle sarcoplasmic reticulum Ca2+-ATPases of chicken and rabbit. Northern blot analysis using a fragment of this cDNA as probe detected a 5.0 kb message in frog skeletal muscle but did not detect any mRNA encoding sarcoplasmic reticulum Ca2+-ATPase in frog cardiac muscle. The enzymatic properties of the amphibian skeletal muscle Ca2+-ATPase were compared with those of the rabbit fast twitch muscle Ca2+-ATPase by functional expression of the cDNAs in COS-1 cells. The amphibian Ca2+-ATPase displayed a reduced apparent affinity for Ca2+ and an increased apparent affinity for the inhibitors, vanadate and thapsigargin, relative to the mammalian enzyme. This may be explained by a mechanism in which relatively more of the E2 conformation accumulated 1n the frog Ca2+-ATPase than in the mammalian enzyme.