PNNL

Abstract

Cyanothece sp. PCC 51142 contains 35 pentapeptide repeat proteins (PRPs), proteins that contain a minimum of eight tandem repeated five-residues (Rfr) of the general consensus sequence A[N/D]LXX. Published crystal structures of PRPs show that the tandem pentapeptide repeats adopt a type of right-handed quadrilateral ß-helix called an Rfr-fold. To characterize how structural features of Rfr-folds vary with different amino acid sequences, the crystal structure of Cyanothece Rfr23 (174 residues) was determined at 2.1 Å resolution. The structure is dominated by an Rfr-fold capped at the N-terminus with a nine-residue a-helix (M26* - E34). The Rfr-fold of Rfr23 contains four structural features previously unobserved in Rfr-folds. First, Rfr23 is composed entirely of type II ß-turns. Second, the pentapeptide repeats are not all tandem in the primary amino acid sequence. Rfr23 contains a 24-residue loop protruding outside one corner of the first complete N-terminal coil of the Rfr-fold (L56 – P79) for which little electron density is observed (24-residue loop). Third, a disulfide bond exists at the corner of one ß-turn in the first coil (disulfide bracket). Size exclusion chromatography and NMR and CD spectroscopy indicate that the reduction of the disulfide bracket with the addition of DTT destroys the entire Rfr-fold. Fourth, a single-residue loop in the C-terminal coil perturbs the last coil slightly about one corner of the Rfr-fold (single-residue loop).