AN EXCEPTIONALLY STABLE HELIX FROM THE RIBOSOMAL-PROTEIN L9 - IMPLICATIONS FOR PROTEIN-FOLDING AND STABILITY

The ribosomal protein L9 has an unusual structure comprising two compa ct globular domains connected by a 34 residue alpha-helix. The middle 17 residues of the helix are exposed to solvent while the first seven pack against and form part of the N-terminal domain, and the last ten form part of the C-terminal domain. Here we report results which show that a peptide corresponding to the central helix of L9 is monomeric i n aqueous solution and >85% helical at 1 degrees C and 68(+7)% helical at 25 degrees C. This is considerably more helical than any other pro tein fragment studied to date. Another peptide corresponding to the mi ddle 17 residues of the helix is monomeric and is 41(+/-4)% helical at 1 degrees C. Because the central helix has high intrinsic stability t he globular N and C-terminal domains will likely be stabilized by thei r interactions with the helix. Therefore, the stability of the two ter minal domains should not be completely independent because both domain s gain stability from a shared structural element, the central helix. Also, the ability of the central helix to form native-like structure i n isolation highlights a potential role for the helix in the early sta ges of the folding process. (C) 1997 Academic Press Limited.