Quinary interactions with an unfolded state ensemble.

Anfinsen's thermodynamic hypothesis states that the native three-dimensional fold of a protein represents the structure with the lowest Gibbs free energy. Changes in the free energy of denaturation can arise from changes to the folded state, the unfolded state, or both. It has been recently recognized that quinary interactions, transient contacts that take place only in cells, can modulate protein stability through interactions involving the folded state. Here we show that the cellular environment can also remodel the unfolded state ensemble.

A cell is more than the sum of its (dilute) parts: A brief history of quinary structure.

Most knowledge of protein structure and function is derived from experiments performed with purified protein resuspended in dilute, buffered solutions. However, proteins function in the crowded, complex cellular environment. Although the first four levels of protein structure provide important information, a complete understanding requires consideration of quinary structure. Quinary structure comprises the transient interactions between macromolecules that provides organization and compartmentalization inside cells. We review the history of quinary structure in the context of several metabolic pathways, and the technological advances that have yielded recent insight into protein behavior in living cells. The evidence demonstrates that protein behavior in isolated solutions deviates from behavior in the physiological environment.

Electrostatic Contributions to Protein Quinary Structure.

There are four well-known levels of protein structure: primary (amino acid sequence), secondary (helices, sheets and turns), tertiary (three-dimensional structure) and quaternary (specific protein-protein interactions). The fifth level remains largely undefined because characterization of quinary structure, the transient but essential macromolecular interactions that organize the crowded cellular interior, requires the measurement of equilibrium thermodynamic parameters in living cells. We have overcome this challenge by quantifying the pH-dependence of quinary interactions in living Escherichia coli cells using the B1 domain of protein G (GB1, 6.2 kDa). To accomplish this goal, we buffered the cellular interior and used NMR-detected amide proton exchange to quantify the free energy of unfolding in cells. At neutral pH, the unfolding free energy in cells is comparable to that in buffered solution. As the pH decreases, the increased number of attractive interactions between E. coli proteins and GB1 destabilizes the protein in cells relative to buffer alone. The data show that electrostatic interactions contribute to quinary structure.

Structure and Inhibition of Microbiome ß-Glucuronidases Essential to the Alleviation of Cancer Drug Toxicity.

The selective inhibition of bacterial ß-glucuronidases was recently shown to alleviate drug-induced gastrointestinal toxicity in mice, including the damage caused by the widely used anticancer drug irinotecan. Here, we report crystal structures of representative ß-glucuronidases from the Firmicutes Streptococcus agalactiae and Clostridium perfringens and the Proteobacterium Escherichia coli, and the characterization of a ß-glucuronidase from the Bacteroidetes Bacteroides fragilis. While largely similar in structure, these enzymes exhibit marked differences in catalytic properties and propensities for inhibition, indicating that the microbiome maintains functional diversity in orthologous enzymes. Small changes in the structure of designed inhibitors can induce significant conformational changes in the ß-glucuronidase active site. Finally, we establish that ß-glucuronidase inhibition does not alter the serum pharmacokinetics of irinotecan or its metabolites in mice. Together, the data presented advance our in vitro and in vivo understanding of the microbial ß-glucuronidases, a promising new set of targets for controlling drug-induced gastrointestinal toxicity.

Intracellular pH modulates quinary structure.

NMR spectroscopy can provide information about proteins in living cells. pH is an important characteristic of the intracellular environment because it modulates key protein properties such as net charge and stability. Here, we show that pH modulates quinary interactions, the weak, ubiquitous interactions between proteins and other cellular macromolecules. We use the K10H variant of the B domain of protein G (GB1, 6.2 kDa) as a pH reporter in Escherichia coli cells. By controlling the intracellular pH, we show that quinary interactions influence the quality of in-cell (15) N-(1) H HSQC NMR spectra. At low pH, the quality is degraded because the increase in attractive interactions between E. coli proteins and GB1 slows GB1 tumbling and broadens its crosspeaks. The results demonstrate the importance of quinary interactions for furthering our understanding of protein chemistry in living cells.

Quinary structure modulates protein stability in cells.

Protein quinary interactions organize the cellular interior and its metabolism. Although the interactions stabilizing secondary, tertiary, and quaternary protein structure are well defined, details about the protein-matrix contacts that comprise quinary structure remain elusive. This gap exists because proteins function in the crowded cellular environment, but are traditionally studied in simple buffered solutions. We use NMR-detected H/D exchange to quantify quinary interactions between the B1 domain of protein G and the cytosol of Escherichia coli. We demonstrate that a surface mutation in this protein is 10-fold more destabilizing in cells than in buffer, a surprising result that firmly establishes the significance of quinary interactions. Remarkably, the energy involved in these interactions can be as large as the energies that stabilize specific protein complexes. These results will drive the critical task of implementing quinary structure into models for understanding the proteome.