Electrostatic contributions to the stability of the GCN4 leucine zipper structure.

Ion pairs are ubiquitous in X-ray structures of coiled coils, and mutagenesis of charged residues can result in large stability losses. By contrast, pK(a) values determined by NMR in solution often predict only small contributions to stability from charge interactions. To help reconcile these results we used triple-resonance NMR to determine pK(a) values for all groups that ionize between pH 1 and 13 in the 33 residue leucine zipper fragment, GCN4p. In addition to the native state we also determined comprehensive pK(a) values for two models of the GCN4p denatured state: the protein in 6 M urea, and unfolded peptide fragments of the protein in water. Only residues that form ion pairs in multiple X-ray structures of GCN4p gave large pK(a) differences between the native and denatured states. Moreover, electrostatic contributions to stability were not equivalent for oppositely charged partners in ion pairs, suggesting that the interactions between a charge and its environment are as important as those within the ion pair. The pH dependence of protein stability calculated from NMR-derived pK(a) values agreed with the stability profile measured from equilibrium urea-unfolding experiments as a function of pH. The stability profile was also reproduced with structure-based continuum electrostatic calculations, although contributions to stability were overestimated at the extremes of pH. We consider potential sources of errors in the calculations, and how pK(a) predictions could be improved. Our results show that although hydrophobic packing and hydrogen bonding have dominant roles, electrostatic interactions also make significant contributions to the stability of the coiled coil.

Partially folded states of staphylococcal nuclease highlight the conserved structural hierarchy of OB-fold proteins.

We have been interested in whether three proteins that share a five-stranded beta-barrel "OB-fold" structural motif but no detectable sequence homology fold by similar mechanisms. Here we describe native-state hydrogen exchange experiments as a function of urea for SN (staphylococcal nuclease), a protein with an OB-fold motif and additional nonconserved elements of structure. The regions of structure with the largest stability and unfolding cooperativity are contained within the conserved OB-fold portion of SN, consistent with previous results for CspA (cold shock protein A) and LysN (anticodon binding domain of lysyl tRNA synthetase). The OB-fold also has the subset of residues with the slowest unfolding rates in the three proteins, as determined by hydrogen exchange experiments in the EX1 limit. Although the protein folding hierarchy is maintained at the level of supersecondary structure, it is not evident for individual residues as might be expected if folding depended on obligatory nucleation sites. Rather, the site-specific stability profiles appear to be linked to sequence hydrophobicity and to the density of long-range contacts at each site in the three-dimensional structures of the proteins. We discuss the implications of the correlation between stability to unfolding and conservation of structure for mechanisms of protein structure evolution.

Molecular basis of coiled-coil formation.

Coiled coils have attracted considerable interest as design templates in a wide range of applications. Successful coiled-coil design strategies therefore require a detailed understanding of coiled-coil folding. One common feature shared by coiled coils is the presence of a short autonomous helical folding unit, termed "trigger sequence," that is indispensable for folding. Detailed knowledge of trigger sequences at the molecular level is thus key to a general understanding of coiled-coil formation. Using a multidisciplinary approach, we identify and characterize here the molecular determinants that specify the helical conformation of the monomeric early folding intermediate of the GCN4 coiled coil. We demonstrate that a network of hydrogen-bonding and electrostatic interactions stabilize the trigger-sequence helix. This network is rearranged in the final dimeric coiled-coil structure, and its destabilization significantly slows down GCN4 leucine zipper folding. Our findings provide a general explanation for the molecular mechanism of coiled-coil formation.

Sensitivity of NMR residual dipolar couplings to perturbations in folded and denatured staphylococcal nuclease.

The invariance of NMR residual dipolar couplings (RDCs) in denatured forms of staphylococcal nuclease to changes in denaturant concentration or amino acid sequence has previously been attributed to the robustness of long-range structure in the denatured state. Here we compare RDCs of the wild-type nuclease with those of a fragment that retains a folded OB-fold subdomain structure despite missing the last 47 of 149 residues. The RDCs of the intact protein and of the truncation fragment are substantially different under conditions that favor folded structure. By contrast, there is a strong correlation between the RDCs of the full-length protein and the fragment under denaturing conditions (6 M urea). The RDCs of the folded and unfolded forms of the proteins are uncorrelated. Our results suggest that RDCs are more sensitive to structural changes in folded than unfolded proteins. We propose that the greater susceptibility of RDCs in folded states is a consequence of the close packing of the polypeptide chain under native conditions. By contrast, the invariance of RDCs in denatured states is more consistent with a disruption of cooperative structure than with the retention of a unique long-range folding topology.

NMR structure of the C-terminal domain of SecA in the free state.

SecA is an integral component of the prokaryotic Sec preprotein secretory translocase system. We report here the solution NMR structure of a fragment corresponding to the C-terminal domain of Escherichia coli SecA. In the presence of Zn2+, the fragment adopts a shortened version of the classic betabetaalpha zinc finger fold. The isolated C-terminal domain shows substantial differences from the X-ray structure of a homologous SecA domain bound to the chaperone-like cofactor SecB. The differences between the structures of the free and bound forms suggest that binding to SecB causes a perturbation of the C-terminal domain's intrinsically favored betabetaalpha fold.