Phospholemman transmembrane structure reveals potential interactions with Na+/K+-ATPase

Phospholemman ( PLM) is a 72- residue bitopic cardiac transmembrane protein, which acts as a modulator of the Na+/K+-ATPase and the Na+/Ca2+ exchanger and possibly forms taurine channels in nonheart tissue. This work presents a high resolution structural model obtained from a combination of site- specific infrared spectroscopy and experimentally constrained high throughput molecular dynamics ( MD) simulations. Altogether, 37 experimental constraints, including nine long range orientational constraints, have been used during MD simulations in an explicit lipid bilayer/ water system. The resulting tetrameric alpha- helical bundle has an average helix tilt of 7.3 degrees and a crossing angle close to 0 degrees. It does not reveal a hydrophilic pore, but instead strong interactions between various residues occlude any pore. The helix- helix packing is unusual, with Gly(19) and Gly(20) pointing to the outside of the helical bundle, facilitating potential interaction with other transmembrane proteins, thus providing a structural basis for the modulatory effect of PLM on the Na+/K+-ATPase. A two- stage model of interaction between PLM and the Na+/K+- ATPase is discussed involving PLM- ATPase interaction and subsequent formation of an unstable PLM trimer, which readily interacts with surrounding ATPase molecules. Further unconstrained MD simulations identified other packing models of PLM, one of which could potentially undergo a conformational transition to an open pore.