Thesis Defense: Yanyan Zhao, Chiu Group

Thesis Defense: Yanyan Zhao, Chiu Group

"Cryo-EM demonstrates that ATP binding to chaperonin mmCpn is statistically random"

Proteins must fold into their correct three-dimensional conformations in order to function properly. Aberrant folding has been linked to a rapidly expanding list of pathologies,  including Alzheimer disease (AD) and Parkinson disease (PD). However, the vast majority of cellular proteins is unable to reach their native state spontaneously. The correct folding of these proteins requires the assistance of a complex cellular machinery of proteins known as molecular chaperones. Among the chaperones, chaperonin is a highly conserved subgroup that consists of two stacked seven- to nine-membered rings. These chaperonins bind nonnative polypeptides in their central cavity and use energy from ATP to mediate the polypeptide folding. So far, ATP induced conformational changes in chaperonins are considered to be coordinated in time and space by complex allosteric regulation. However, the current models employed to describe allostery in chaperonin, does not distinguish allostery in ATP binding from that in ATP hydrolysis.

Here I use cryo-EM to study the ATP occupancy and distribution in each ring of the mmCpn chaperonin complex. By exploring the nucleotide distribution in hundreds of thousands of mmCpn subunits, I find that ATP binding to chaperonin mmCpn is statistically random both within the ring and across the ring. The quaternary structure of mmCpn differs from particle to particle and the lid closing of the two rings is found to be uncorrelated to each other. Our observation of a high subunit conformational and compositional heterogeneity in each ring of mmCpn complex suggests that intra-ring subunit conformational change is non-concerted and the existence of intermediate states may help explain the chaperonin’s ability of folding very large protein substrates.