Cytosolic chaperonin‐containing t‐complex polypeptide 1 changes the content of a particular subunit species concomitant with substrate binding and folding activities during the cell cycle

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Abstract
The chaperonin‐containing t‐complex polypeptide 1 (CCT) is a cytosolic molecular chaperone composed of eight subunits that assists in the folding of actin, tubulin and other cytosolic proteins. We show here that the content of particular subunits of CCT within mammalian cells decreases concomitantly with the reduction of chaperone activity during cell cycle arrest at M phase. CCT recovers chaperone activity upon resumption of these subunits after release from M phase arrest or during arrest at S phase. The levels of α, δ and ζ‐1 subunits decreased more rapidly than the other subunits during M phase arrest by colcemid treatment and recovered after release from the arrest. Gel filtration chromatography or native (nondenaturing) PAGE analysis followed by immunoblotting indicated that the α and δ subunit content in the 700‐ to 900‐kDa CCT complex was appreciably lower in the M phase cells than in asynchronous cells. In vivo, the CCT complex of M‐phase‐arrested cells was found to bind lower amounts of tubulin than that of asynchronous cells. In vitro, the CCT complex of M phase‐arrested cells was less active in binding and folding denatured actin than that of asynchronous cells. On the other hand, the CCT complex of asynchronous cells (a mixture of various phases of cell cycle) exhibited lower α and δ subunit content and lower chaperone activity than that of S‐phase‐arrested cells obtained by excess thymidine treatment. In addition, turnover (synthesis and degradation) rates of the α and δ subunits in vivo were more rapid than those of most other subunits. These results suggest that the content of α and δ subunits of CCT reduces from the complete active complex in S phase cells to incomplete inactive complex in M phase cells.

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