Association with SERCA2a directs phospholamban trafficking to sarcoplasmic reticulum from a nuclear envelope pool

Phospholamban (PLB) stoichiometrically regulates the cardiac Ca²⁺ pump (SERCA2a) in the sarcoplasmic reticulum (SR); but in the nuclear envelope (NE) of cardiomyocytes (CMs), the PLB to SERCA2a molar ratio is higher, which highlights our poor understanding of how SR proteins distribute to their functional subcompartments. By tracking newly made PLB and SERCA2a in CMs, we will elucidate underlying cellular pathways responsible for their unique intracellular distributions.

Highly specific monoclonal antibodies were used to compare the subcellular distributions of SERCA2a, PLB, and junctin (JCN) in dog heart tissue. The data supported a view that both non-junctional and junctional SR proteins are all prominently enriched in transverse stretches of SR tubules, along the edges of sarcomeres (SR z-tubules). To understand the genesis of these steady state distributions, we analyzed confocal immunofluorescence images of adult rat CMs after acute expression (12-48 h) of the dog ortholog of PLB (dPLB) or dSERCA2a. Newly made dog proteins in rat CMs were detected using dog-specific monoclonal antibodies. By 12-24 h, dSERCA2a had accumulated within the NE in a punctate pattern, presumably reflecting initial sites of biosynthesis. Over the next 24-48 h, higher levels of dSERCA2a immunofluorescence accumulated in transverse/radial SR tubules, aligned along sarcolemmal transverse (T)-tubules, and extending from NE puncta. The patterns of SR tubules carrying dSERCA2a overlapped with those for newly made JCN, suggesting a common Nuclear Envelope to SR along T-tubules or NEST pathway for SR proteins. In contrast to the SERCA2a distribution pattern, dPLB accumulated uniformly in the NE, without visible puncta. With co-expression of dSERCA2a, however, PLB no longer uniformly filled the NE, but instead moved together with SERCA2a to form bright NE puncta, from which the two proteins then trafficked anterogradely.

Expression of dog SR protein orthologs (dSERCA2a, dPLB, and dJCN) for as little as 48 h reproduces their characteristic steady state distributions. Detailed analyses of the time courses of protein accumulation suggest a possible mechanism by which PLB distributes to both the NE and SR, unlike SERCA2a. SERCA2a moves in SR z-tubules directly from rough ER, along pathways that are in common with those used by junctional SR proteins. A different trafficking route for PLB away the rough ER/NE led to its accumulation in the NE, a process that may account for its enrichment in NE in situ. Association of SERCA2a with PLB from this NE pool enhanced PLB trafficking along the NEST pathway, contributing to steady state stoichiometry and physiologically regulated SERCA2a.