ABSTRACT Protein folding in the cell is critically dependent on the assistance of molecular chaperones. The ring-shaped chaperonins are essential members of the cellular folding machinery. These large protein complexes consist of two stacked seven- to nine-membered rings. Chaperonins bind unfolded substrates in their central cavity and use binding and hydrolysis of ATP to mediate polypeptide folding. Substrate proteins are thought to fold upon encapsulation in the central cavity formed by each ring. The long term goal of this program is to understand how the chaperonin of eukaryotic cells, TRiC, mediates polypeptide folding., TRiC is hetero-oligomeric and uses ATP cycling to open and close a built-in lid over the central chamber. Intriguingly, TRiC has the ability to fold some eukaryotic proteins, such as actin, that cannot be folded by any other chaperone. Despite its essential role in cellular folding, little is known about the mechanism and substrate binding properties of TRiC. Our work in the previous funding period provided important mechanistic and structural insights into this chaperonin. We established that the conformational cycle of TRiC is significantly different from that of bacterial chaperonins and demonstrated subunit diversity confers dramatic functional asymmetry to this seemingly symmetric chaperonin. Importantly, in the previous funding period we developed a recombinant system to study this chaperonin that for the first time makes it possible to generating mutants that can be studies functionally and structurally. These breakthroughs enable us to study how TRiC facilitates folding in unprecedented detail, through the following proposed aims: 1. Characterize of the nucleotide cycle of the chaperonin TRiC: Chaperonins use ATPase cycling to promote conformational changes leading to protein folding. We want to understand how the ATPase cycle of TRiC is coordinated among the different TRiC subunits, and how ATP cycling drives conformational changes in the chaperonin and how the "built-in" lid that opens and closes in response to ATP-binding and hydrolysis. 2. Define the molecular basis of TRiC-substrate interactions: Little is known about the molecular basis of TRiC-substrate interactions. We want to define the substrate recognition code of the binding sites for the different subunits in the chaperonin and define the motifs within substrates that are recognized by TRiC. 3. Investigate the mechanism of TRiC-assisted substrate folding: The exact role that chaperonins play with respect to the substrate is still a mystery. We will explore the effect of TRiC on substrate proteins during the different stages of the folding cycle by combining biochemical approaches together with crosslinking and fluorescence spectroscopy. Importantly, recent observations have highlighted the links between TRiC and several pathological states incuding cancer, viral infection and neurodegeneration. Thus our project deciphering the mechanism of this chaperoni...