PNNL

Abstract

A unique feature of cyanobacteria genomes is the abundance of genes that code for hypothetical proteins containing tandem pentapeptide repeats approximately described by the consensus motif A[N/D]LXX. Too date, structures of two pentapeptide repeat proteins (PRPs) have been determined with the tandem pentapeptide repeat sequences observed to adopt a novel right-handed quadrilateral b-helix, or Rfr-fold, in both structures. One structure, Mycobacterium tuberculosis MfpA, is a 183-residue protein that contains 30 consecutive pentapeptide repeats and appears to offer antibiotic resistance by acting as a DNA mimic. The other structure, Cyanothece Rfr32, is a 167-residue protein that contains 21 consecutive pentapeptide repeats. The function of Rfr32, like the other 35 hypothetical PRPs identified in the genome of Cyanothece, is unknown. In an effort to understand the role of PRPs in cyanobacteria, and to better characterize the structural properties of Rfr-folds with different amino acid sequences, a second PRP from Cyanothece 51142, Rfr23, has been cloned, expressed, and purified. Selenomethione substituted protein was crystallized by vapor diffusion in hanging drops. MAD diffraction data were collected on these crystals to 2.? Å resolution using synchrotron radiation. The crystals belonged to space group I41 with unit-cell parameters a = b = 106.23 Å, c = 52.40 Å. Analysis of the 172-residue protein sequence suggests that Rfr23 contains 26 pentapeptide repeats interrupted by eight residues near the N-terminus. The electron density map suggests that the pentapeptide repeats adopt a similar right-handed quadrilateral b-helix as observed in the other two PRP structures, however, the eight residue interruption in the string of pentapeptide repeats appears to create a distortion in the Rfr-fold.