Type I beta turns make a new twist in pentapeptide repeat proteins: Crystal structure of Alr5209 from Nostoc sp. PCC 7120 determined at 1.7 angstrom resolution

Pentapeptide repeat proteins (PRPs) are found abundantly in cyanobacteria, numbering in the dozens in some genomes, e.g. in Nostoc sp. PCC 7120. PRPs, comprised of a repeating consensus sequence of five amino acids, adopt a distinctive right-handed quadrilateral beta-helical structure, also referred to as a repeat five residue (Rfr) fold, made up of stacks of coils formed by four consecutive pentapeptide repeats. The right-handed quadrilateral beta-helical PRP structure is constructed by repeating beta turns at each of four corners in a given coil, each causing a 90 degrees change in direction of the polypeptide chain. Until now, all PRP structures have consisted either of type II and IV beta turns or exclusively of type II beta turns. Here, we report the first structure of a PRP comprised of type I and II beta turns, Alr5209 from Nostoc sp. PCC 7120. The alr5209 gene encodes 129 amino acids containing 16 tandem pentapeptide repeats. The Alr5209 structure was analyzed in comparison to all other PRPs to determine how type I beta turns can be accommodated in Rfr folds and the consequences of type I beta turns on the right-handed quadrilateral beta-helical structure. Given that Alr5209 represents the first PRP structure containing type I beta turns, the PRP consensus sequence was reevaluated and updated. Despite a growing number of PRP structural investigations, their function remains largely unknown. Genome analysis indicated that alr5209 resides in a five-gene operon (alr5208-alr5212) with Alr5211 annotated to be a NADH dehydrogenase indicating Alr5209 may be involved in oxidative phosphorylation.