Regulation of the Sarcoplasmic Reticulum Calcium Pump by Divergent Phospholamban Isoforms in Zebrafish [Membrane Biology]

January 15th, 2015 by Gorski, P. A., Trieber, C. A., Ashrafi, G., Young, H. S.

The sarcoplasmic reticulum calcium pump (SERCA) is regulated by small integral membrane proteins phospholamban (PLN) and sarcolipin (SLN). These regulators have homologous transmembrane regions, yet they differ in their cytoplasmic and luminal domains. While the sequences of PLN and SLN are practically invariant amongst mammals, they vary in fish. Zebrafish appear to harbor multiple PLN isoforms, one of which contains 18 sequence variations and a unique luminal extension. Characterization of this isoform (zfPLN) revealed that SERCA inhibition and reversal by phosphorylation were comparable to human PLN. To understand the sequence variations in zfPLN, chimeras were created by transferring the N-terminus, linker and C-terminus of zfPLN onto human PLN. A chimera containing the N-terminal domain resulted in mild loss of function, while a chimera containing the linker domain resulted in gain of function. This latter effect was due to changes in basic residues in the linker region of PLN. Removing the unique luminal domain of zfPLN (53SFHGM) resulted in loss of function, while adding this domain to human PLN had a minimal effect on SERCA inhibition. We conclude that the luminal extension contributes to SERCA inhibition, but only in the context of zfPLN. Although this domain is distinct from the SLN luminal tail, zfPLN appears to use a hybrid PLN-SLN inhibitory mechanism. Importantly, the different zebrafish PLN isoforms raises the interesting possibility that SR calcium handling and cardiac contractility may be regulated by the differential expression of PLN functional variants.