Flexible zinc finger requirement for binding of the transcriptional activator staf to U6 small nuclear RNA and tRNA(Sec) promoters.

The transactivator Staf, which contains seven zinc finger motifs, exerts its effect on gene expression by binding to specific targets in small nuclear RNA (snRNA) and snRNA-type gene promoters. In this work, binding site selection allowed us to identify the 21-base pair ATTACCCATAATGCATYGCGG sequence as the high affinity consensus binding site for Staf. It shows a high sequence divergence with Staf-responsive elements in the Xenopus selenocysteine tRNA (tRNA(Sec)) and human U6 snRNA promoters. By using a combination of approaches, we analyzed the interaction of wild-type and truncated Staf zinc finger domains with the consensus, Xenopus tRNA(Sec), and human U6 sites. Two main conclusions emerged from our data. First, the data clearly indicate that zinc finger 7 does not establish base-specific contacts in Staf-DNA complexes. The second conclusion concerns zinc finger 1, which is required for the binding to the Xenopus tRNA(Sec) site but is dispensable in the case of the human U6 site. Taking into account the sequence differences in the two sites, these findings demonstrate that Staf utilizes zinc finger 1 in a rather flexible manner, illustrating how a protein can interact with DNAs containing targets of different sequences.     

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.

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 alpha, delta and zeta-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 alpha and delta 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 alpha and delta 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 alpha and delta subunits in vivo were more rapid than those of most other subunits. These results suggest that the content of alpha and delta subunits of CCT reduces from the complete active complex in S phase cells to incomplete inactive complex in M phase cells.