Effects of a polar amino acid substitution on helix formation and aggregate size along the detergent-induced peptide folding pathway

Abstract

Membrane proteins constitute a significant fraction of the proteome and are important drug targets. While the transmembrane (TM) segments of these proteins are primarily composed of hydrophobic residues, the inclusion of polar residues-either naturally occurring or as a consequence of a disease-related mutation-places a significant folding burden in this environment, potentially impacting bilayer insertion and/or association of neighboring TM helices. Here we investigate the role of an anionic detergent, sodium dodecylsulfate (SDS), and a zwitterionic detergent, dodecylphosphocholine (DPC), in the folding process, and the effects induced by a single polar substitution, on structure and topology of model α-helical TM segments. The peptides, represented by KK-YAAAIAAIAWAXAAIAAAIAA-KKK-NH(2), where X is I or N, are designed with high aqueous solubilities, through poly-lysine tags. Circular dichroism (CD) and NMR were used to monitor peptide secondary structure and diffusional mobility of both peptide and the detergent hosts. For both peptides, SDS binding commenced at a concentration below its CMC, due to Coulombic attraction of anionic SDS to cationic Lys residues. Increasing SDS binding correlated with increasing peptide helicity. Pulsed field gradient (PFG) NMR diffusion measurements revealed that the Asn-containing peptide bound four fewer detergent molecules, corresponding to ca. 20% less SDS than bound by the Ile peptide. Conversely, zwitterionic DPC binding to either peptide was not observed until the DPC concentration approached its CMC. Our findings confirm quantitatively that a single polar residue within a TM segment may have a significant influence on its local membrane environment.