SURFACE-EXPOSED PHENYLALANINES IN THE RNP1 RNP2 MOTIF STABILIZE THE COLD-SHOCK PROTEIN CSPB FROM BACILLUS-SUBTILIS/

In the cold-shock protein CspB from Bacillus subtilis three exposed Ph e residues (F15, F17, and F27) are essential for its function in bindi ng to single-stranded nucleic acids. Usually, the hydrophobic Phe side chains are buried in folded proteins. We asked here whether the expos ition of the essential Phe residues could be a cause for the very low conformational stability of CspB. Urea-induced and heat-induced equili brium unfolding transitions were measured for three mutants of CspB, w here Phe 15, Phe 17, and Phe 27 were individually replaced by alanine. Unexpectedly, all three mutations strongly destabilized CspB. The aro matic side chains of Phe 15, Phe 17, and Phe 27 in the active site are thus important for both binding to nucleic acids and conformational s tability. There is no compromise between function and stability in the active site. Model calculations indicate that, although they are part ially exposed to solvent, all three Phe residues nevertheless lose acc essible surface upon folding, and this should favor the native state. A different result is obtained with the F38A variant. Phe 38 is hypere xposed in native CspB, and its substitution by Ala is in fact stabiliz ing. (C) 1998 Wiley-Liss, Inc.