Abstract 282: Phospholamban Truncation Mutations Including Heart Failure Mutant L39stop Disrupt Membrane Localization.

Abstract

Introduction: Phospholamban (PLB) is an integral sarcoplasmic reticulum (SR) membrane protein, which directly regulates cardiac Ca 2+ handling and contractility by reversibly inhibiting SR Ca 2+ ATPase (SERCA). Our previous studies have suggested that the naturally occurring human heart failure mutation of PLB, L39X disrupts membrane localization. Hypothesis: We hypothesize that the membrane localization of PLB is a prerequisite for PLB oligomerization and interaction with SERCA. The truncation mutations in C-terminus of PLB will disrupt membrane localization, PLB oligomerization, and SERCA regulation. Results and Methods: To identify the minimum length of PLB required for membrane localization and function, we generated a series of C-terminal transmembrane truncation mutants of PLB (tagged N-terminally with Cer or YFP) including L51X, M50X, V49X, I48X, I38X, I33X, and the heart-failure mutant L39X. Confocal microscopy revealed that progressive truncation of the C-terminal residues of PLB resulted in escalating increase in mislocalization of PLB to the cytoplasm and nucleus. In addition, we observed an increased solubilization of PLB as indicated by loss of YFP fluorescence after selective permeabilization of the plasma membrane by saponin. As expected, there was no change in localization of Cer-SERCA upon saponin permeabilization. Next, western blot analysis exhibited a decrease in molecular weight corresponding to the relative sizes of truncation mutants compared to full length PLB, indicating that protein degradation is not the cause of membrane mislocalization. Fluorescence resonance energy transfer analysis revealed that truncating the C-terminal residues of PLB results in a progressive decrease in apparent affinity of PLB oligomerization and interaction with SERCA. Finally, molecular dynamics simulations exhibited that the heart failure mutant L39X was unstable compared to full length PLB pentamer and started protruding out of the bilayer until complete solubilization. Conclusions: Truncating only two C-terminal residues of PLB resulted in significant mislocalization, while deleting five or more residues profoundly disrupted membrane localization, PLB oligomerization and SERCA regulation.