Two-dimensional solid-state NMR reveals two topologies of sarcolipin in oriented lipid bilayers

Sarcolipin (SLN), a 31 amino acid integral membrane protein, regulates SERCA1a and SERCA2a, two isoforms of the sarco(endo)plasmic Ca-ATPase, by lowering their apparent Ca²⁺ affinity and thereby enabling muscle relaxation. SLN is expressed in both fast-twitch and slow-twitch muscle fibers with significant expression levels also found in the cardiac muscle. SLN shares ∼30% identity with the transmembrane domain of phospholamban (PLN), and recent solution NMR studies carried out in detergent micelles indicate that the two polypeptides bind to SERCA in a similar manner. Previous ID solid-state NMR experiments on selectively ¹⁵N-labeled sites showed that SLN crosses the lipid bilayer with an orientation nearly parallel to the bilayer normal. With a view toward the characterization of SLN structure and its interactions with both lipids and SERCA, herein we report our initial structural and topological assignments of SLN in mechanically oriented DOPC/DOPE lipid bilayers as mapped by 2D ¹⁵N PISEMA experiments. The PISEMA spectra obtained on uniformly ¹⁵N-labeled protein as well as ¹⁵N-Leu, ¹⁵N-Ile and ¹⁵N-Val map the secondary structure of SLN and, simultaneously, reveal that SLN exists in two distinct topologies. Both the major and the minor populations assume an orientation with the helix axis tilted by ∼23° with respect to the lipid bilayer normal, but vary in the rotation angle about the helix axis by ∼5°. The existence of the multiple populations in model membranes may be a significant requirement for SLN interaction with SERCA.