Variation of phospholamban in slow-twitch muscle sarcoplasmic reticulum between mammalian species and a link to the substrate specificity of endogenous Ca2+-calmodulin-dependent protein kinase

Systematic immunological and biochemical studies indicate that the level of expression of sarcoplasmic reticulum (SR) Ca2+-ATPase regulatory protein p hospholamban (PLB) in mammalian slow-twitch fibers varies from zero, in the rat, to significant levels in the rabbit, and even higher in humans. The l ack of PLB expression in the rat, at the mRNA level, is shown to be exclusi ve to slow-twitch skeletal muscle, and not to be shared by the heart, thus suggesting a tissue-specific, in addition to a species-specific regulation of PLB. A comparison of sucrose density-purified SR of rat and rabbit slow- twitch muscle, with regard to protein compositional and phosphorylation pro perties, demonstrates that the biodiversity is two-fold, i.e. (a) in PLB me mbrane density; and (b) in the ability of membrane-bound Ca2+-calmodulin (C aM)-dependent protein kinase II to phosphorylate both PLB and SERCA2a (slow -twitch isoform of Ca2+-ATPase). The basal phosphorylation state of PLB at Thr-17 in isolated SR vesicles from rabbit slow-twitch muscle, colocalizati on of CaM K II with PLB and SERCA2a at the same membrane domain, and the di vergent subcellular distribution of PKA, taken together, seem to argue for a differential heterogeneity in the regulation of Ca2+ transport between su ch muscle and heart muscle. (C) 2000 Elsevier Science B.V. All rights reser ved.