Abstract by Michael Mann
Chemistry and Biochemistry
Structural and functional analysis of the role of the chaperonin CCT in mTOR complex assembly
The mechanistic target of rapamycin (mTOR) kinase forms two multi-protein signaling complexes, mTORC1 and mTORC2, which are master regulators of cell growth, metabolism, survival and autophagy. Two of the main subunits of these complexes are mLST8 and Raptor, beta-propeller proteins that stabilize the mTOR kinase domain and recruit substrates, respectively. We have found that the eukaryotic chaperonin CCT plays a key role in mTORC assembly and signaling by folding both mLST8 and Raptor. A high resolution (3.97 angstroms) cryo-EM structure of the human mLST8-CCT intermediate isolated directly from cells shows an almost native mLST8 bound to CCT deep within the folding chamber between the two CCT rings. This structure reveals a unique binding site in CCT for mLST8, far from those found for similar beta-propeller proteins, which are near the top of the folding cavity. Moreover, mLST8 associates on the side of CCT that harbors subunits with poor nucleotide release activity, suggesting a correlation between substrate release and ATP utilization.