Conformational studies of the C-terminal domain of bacteriophage Pf1 gene 5 protein

The gene 5 protein (g5p) of the bacteriophage Pf1 is a 144 residue single-s tranded (ss) DNA binding protein involved in replication and packaging of t he viral DNA. Compared to the gene 5 proteins of other filamentous bacterio phages, such as fd, the Pf1 g5p has an additional C-terminal sequence (simi lar to 40 residues) with an unusual amino acid composition, being particula rly rich in proline, glutamine and alanine. This C-terminal sequence is sus ceptible to limited proteolysis, in contrast to the globular N-terminal dom ain of the protein. The C-terminal sequence has been shown to play a role i n the stabilisation of the protein-ssDNA complex. In the present study, the DNA sequence corresponding to the 38 amino acid residue C-terminal peptide has been cloned and expressed. A variety of biophysical techniques suggest that this peptide has a largely irregular conformation in solution, in con trast to the N-terminal globular domain that is principally beta-sheet. How ever, circular dichroism (CD) spectroscopy indicates that the peptide can b e induced to form a structure that resembles a left-handed polyproline-like (P-II) helix, suggesting that the C-terminal tail of the protein may adopt a more structured conformation in the appropriate physiological environmen t. (C) 1999 Elsevier Science B.V. All rights reserved.