Proline kinks in transmembrane α-helices

Integral membrane proteins often contain proline residues in their presumably α-helical transmembrane segments. This is in marked contrast to globular proteins, where proline is rarely found inside α-helices. Proline residues cause kinks in helices, and, in addition to leaving the i — 4 backbone carbonyl without its normal hydrogen bond donor, also sterically prevent the (i — 3)-carbonyl-(i + 1)-amide backbone hydrogen bond from forming. Here, some structural aspects of proline kinks in transmembrane helices are discussed on the basis of an analysis of Pro-kinked helices in the photosynthetic reaction center and bacteriorhodopsin, as well as results from an analysis of Pro-containing transmembrane segments identified in the NBRF Protein Sequence Databank.