Sarcolipin, the Shorter Homologue of Phospholamban, Forms Oligomeric Structures in Detergent Micelles and in Liposomes

The human 31-amino acid integral membrane protein sarcolipin (SLN), which regulates the sarcoplasmic reticulum Ca-ATPase in fast-twitch skeletal muscle, was chemically synthesized. Appropriate synthesis and purification strategies were used to achieve high purity and satisfactory yields of this hydrophobic and poorly soluble protein. Structural and functional properties of SLN were analyzed and compared with the homologous region of human phospholamban (PLB) comprising residues Ala²⁴-Leu⁵² (PLB-(24-52)), the regulatory protein of the cardiac sarcoplasmic reticulum Ca-ATPase. Circular dichroism spectroscopy showed that SLN is a predominantly α-helical protein and that the secondary structure is highly resistant to SDS and thermal denaturation. In this respect SLN is remarkably similar to PLB-(24-52). However, SLN is monomeric in SDS gels, whereas PLB-(24-52) shows a monomer-pentamer equilibrium typical for native PLB. Analytical ultracentrifugation experiments revealed that SLN oligomerizes in the presence of the nonionic detergents octylpolyoxyethylene and octyl glucoside in a concentration-dependent manner. No plateau was observed, and a pentameric state was only reached at much higher protein concentrations compared with PLB-(24-52). Chemical cross-linking showed that also in liposomes SLN has the ability to self-associate to oligomers. PLB-(24-52) specifically oligomerized to pentamers in the presence of octylpolyoxyethylene as well as in liposomes at low protein concentrations. In the presence of octylpolyoxyethylene pentamers were the main oligomeric species, whereas in liposomes monomers and dimers were predominant. Increasing the protein concentration led to self-association of PLB-(24-52) pentamers in the presence of octylpolyoxyethylene. Functional reconstitution of Ca-ATPase with PLB-(24-52) and SLN in liposomes showed that both proteins regulate the Ca-ATPase in a similar manner.