Elucidation of the subunit orientation in CCT (chaperonin containing TCP1) from the subunit composition of CCT micro-complexes.

A collection of chaperonin containing TCP1 (CCT) micro-complexes that are comprised of subsets of the constitutively expressed CCT subunits have been identified. These CCT micro-complexes have mol. wts ranging from 120 to 250 kDa and are present in cells at lower abundance (<5%) as compared with intact CCT. Biochemical characterization of these microcomplexes has shown that several are comprised of two different types of CCT subunit. Furthermore, it was observed that each subunit associates with only one or two other different types of subunit, suggesting that each subunit has fixed partners. This observation, together with CCT gene counting being concordant with the 8-fold structural symmetry, is consistent with predictions derived from analysis of the primary structures of these subunits concerning inter-subunit interactions, and implies a unique topology of the subunits constituting the torodial ring in CCT. The series of subunit-subunit association patterns determined from CCT micro-complexes has provided information to infer, from the 5040 (7!factorial) combinatorial possibilities, one probable subunit orientation within the torodial ring.     

Increased expression of cytosolic chaperonin CCT in human hepatocellular and colonic carcinoma

The chaperonin-containing t-complex polypeptide 1 (CCT) is a hetero-oligomeric molecular chaperone that assists in the folding of actin, tubulin, and other cytosolic proteins. We recently reported that the expression level of CCT is closely correlated with growth rates of mammalian cultured cells. Here we examine the levels of CCT subunits and other molecular chaperones in tumor tissues of patients with hepatocelluar and colonic carcinoma, and compare them with nontumor tissues in the same patients. Expression levels of CCTβ in tumor tissues was significantly higher than in nontumor tissues in all patients with hepatocellular carcinoma (n = 15) and 83% of patients with colonic carcinoma (n = 17). The increased level of CCT expression in colonic cancer cells was confirmed by immunohistochemistry with anti-CCTβ antibody. The levels of CCTβ were highly correlated (r = 0.606) with those of the proliferating cell nuclear antigen (PCNA), which was used as an indicator of cell growth. CCTα gave similar results, although the correlation with PCNA levels was weaker. Other cytosolic and endoplasmic reticulum chaperones also showed higher expression in significant numbers of tumor tissues but less frequently than that observed with CCT. These results suggest that CCT is up-regulated in rapidly proliferating tumor cells in vivo to effectively produce proteins required for growth, and may serve as a useful tumor marker because it is widely distributed in the cytosol.     

Structure and function of a protein folding machine: the eukaryotic cytosolic chaperonin CCT

Coupled translocation of tRNA and mRNA in the ribosome during protein synthesis is one of the most challenging and intriguing problems in the field of translation. We highlight several key questions regarding the mechanism of translocation, and discuss possible mechanistic models in light of the recent crystal structures of the ribosome and its subunits.