Thursday 12 May 2016 at 10:00am

Title: “Structure and Function of the Group III Chaperonins, A Unique Clade of Protein Folding Nanomachines”

Speaker:
Sara Rowland, MEES Graduate Program

Abstract:
The survival and descent of cells is universally dependent on maintaining their proteins in a properly folded condition. It is widely accepted that the information for the folding of the nascent polypeptide chain into a native protein is encrypted in the amino acid sequence, and the Nobel Laureate Christian Anfinsen was the first to demonstrate that a protein could spontaneously refold after complete unfolding. However, it became clear that the intrinsic folding rates for many proteins was much slower than rates observed in vivo. This led to the recognition of required protein-protein interactions that promote proper folding. A unique group of proteins, the molecular chaperones, are primarily responsible for maintaining protein homeostasis, during normal growth as well as stress conditions.

Chaperonins (CPNs) are ubiquitous and essential chaperones. They form ATP-dependent, hollow complexes that encapsulate polypeptides in two back-to-back stacked multisubunit rings, facilitating protein folding actively through highly cooperative allosteric articulation. CPNs are usually classified into Group I and Group II. Here, I report the characterization of a novel CPN, belonging to a novel third Group, recently discovered in the extremely thermophilic bacterium, Carboxydothermus hydrogenoformans. GroupIII CPNs have close phylogenetic affinity to the GroupII CPNs found in Archaea and Eukarya, and may be a relic of the Last Common Ancestor of the CPN family.

The gene encoding the GroupIII CPN was cloned in E. coli and overexpressed in order to both characterize the protein and to demonstrate its ability to function as a chaperone. The opening and closing cycle of the chaperone was examined via site-directed mutations affecting the ATP binding sites and the opening and closing of the complex. To relate the mutational analysis to the structure of the CPN, the crystal structure of both the AMPPNP (an ATP analogue) and ADP bound forms were obtained in collaboration with Sun-Shin Cha in Seoul, South Korea. The ATP and ADP binding site substitutions resulted in frozen forms of the structures in open and closed conformations. From this, mutational analysis was designed to validate hypotheses regarding the mechanism of closure and to observe the physical properties of the complexes by calorimetry.

Host: Dr. Frank Robb, Ph.D.