U. Mueller et al., Thermal stability and atomic-resolution crystal structure of the Bacillus caldolyticus cold shock protein, J MOL BIOL, 297(4), 2000, pp. 975-988
The bacterial cold shock proteins are small compact beta-barrel proteins wi
thout disulfide bonds, cis-proline residues or tightly bound cofactors. Bc-
Csp, the cold shock protein from the thermophile Bacillus caldolyticus show
s a twofold increase in the free energy of stabilization relative to its ho
molog Bs-CspB from the mesophile Bacillus subtilis, although the two protei
ns differ by only 12 out of 67 amino acid residues. This pair of cold shock
proteins thus represents a good system to study the atomic determinants of
protein thermostability. Bs-CspB and Bc-Csp both unfold reversibly in coop
erative transitions with T-M values of 49.0 degrees C and 77.3 degrees C, r
espectively, at pH 7.0. Addition of 0.5 M salt stabilizes Bs-CspB but desta
bilizes Bc-Csp. To understand these differences at the structural level, th
e crystal structure of Bc-Csp was determined at 1.17 Angstrom resolution an
d refined to R = 12.5 % (R(free) = 17.9 %). The molecular structures of Bc-
Csp and Bs-CspB are virtually identical in the central P-sheet and in the b
inding region for nucleic acids. Significant differences are found in the d
istribution of surface charges including a sodium ion binding site present
in Bc-Csp, which was not observed in the crystal structure of the Bs-CspB.
Electrostatic interactions are overall favorable for Bc-Csp, but unfavorabl
e for Bs-CspB. They provide the major source for the increased thermostabil
ity of Bc-Csp. This can be explained based on the atomic-resolution crystal
structure of Bc-Csp. It identifies a number of potentially stabilizing ion
ic interactions including a cation-binding site and reveals significant cha
nges in the electrostatic surface potential. (C) 2000 Academic Press.