In metazoans, certain calmodulin-related calcium-binding proteins (recoverins, neurocalcins and frequenins) are found at highest levels in excitable cells, but their physiological roles are largely uncharacterized. Here we show that Saccharomyces cerevisiae contains a frequenin homologue, Frq1, and that its target is Pik1, a phosphatidylinositol-4-OH kinase. Frq1 binds to a conserved sequence motif in Pik1 outside Pik1's catalytic domain and stimulates its activity in vitro. N-myristoylated Frq1 may also assist in Pik1 localization.

Human BRG1, a subunit of the Swi/Snf chromatin remodeling apparatus, has been implicated in regulation of cellular proliferation and is a candidate tumor suppressor. Reintroduction of BRG1 into a breast tumor cell line, ALAB, carrying a defined mutation in the BRG1 gene, induced growth arrest. Gene expression data revealed that the arrest may in part be accounted for by down-regulation of select E2F target genes such as cyclin E, but more dramatically, by up-regulation of mRNAs for the cyclin-dependent kinase inhibitors p21 and p15. Protein levels of both p15 and p21 were induced, and p21 protein was recruited to a complex with cyclin-dependent kinase, CDK2, to inhibit its activity. BRG1 can associate with the p21 promoter in a p53-independent manner, suggesting that the induction of p21 by BRG1 may be direct. Further, using microarray and real-time PCR analysis we identified several novel BRG1-regulated genes. Our work provides further evidence for a role for BRG1 in the regulation of several genes involved in key steps in tumorigenesis and has revealed a potential mechanism for BRG1-induced growth arrest.The evolutionarily conserved Swi/Snf chromatin remodeling apparatus has functions in both transcriptional activation and repression (29,36,55). The Swi/Snf complex has been shown to alter the arrangement of nucleosomes along DNA in an ATPdependent manner in vitro, thus allowing access for transcription factors to bind their target DNA sequences (29). The mammalian Swi/Snf complex is a 1.5-to 2.0-MDa multisubunit complex, the ATPase activity being generated by BRG1, a 190-kDa component, or a related protein, hBRM (58).In mammalian cells BRG1 is known to play a role in the regulation of cellular proliferation (8,49). This activity of BRG1 has been shown to be dependent on its association with the retinoblastoma tumor suppressor gene product (pRB) and, consequently, via the repression of E2F target gene expression (29). Specifically, it was demonstrated that BRG1/pRB complexes repress certain E2F target genes such as cyclin E, cyclin A, and CDC2 (48, 61). The temporally ordered association of BRG1 with histone deacetylase and pRB complexes ensures sequential activation of cyclin E and then cyclin A and maintains the order of G 1 and S phases of the cell cycle (61).Studies have now revealed that BRG1 is frequently deleted or mutated in a variety of tumor cell lines (59), implicating BRG1 as a potential tumor suppressor gene in its own right. The tumor suppressor function of BRG1 is supported by the generation of mice harboring a BRG1 null mutation. Heterozygotes for BRG1 are susceptible to neoplasia and display large subcutaneous tumors, suggesting that a partial reduction in cellular BRG1 level leads to critical loss of proliferation control (5). More recent work has shown loss of expression of BRG1/BRM expression in a subset of primary lung tumors associated with poor prognosis (39). Several studies have also revealed that the INI1/SNF5 conserved component of the SWI/ SNF complex is also mutated in human tumors,...

The FRQ1 gene is essential for growth of budding yeast and encodes a 190-residue, N-myristoylated (myr) calcium-binding protein. Frq1 belongs to the recoverin/frequenin branch of the EF-hand superfamily and regulates a yeast phosphatidylinositol 4-kinase isoform. Conformational changes in Frq1 due to N-myristoylation and Ca(2+) binding were assessed by nuclear magnetic resonance (NMR), fluorescence, and equilibrium Ca(2+)-binding measurements. For this purpose, Frq1 and myr-Frq1 were expressed in and purified from Escherichia coli. At saturation, Frq1 bound three Ca(2+) ions at independent sites, which correspond to the second, third, and fourth EF-hand motifs in the protein. Affinity of the second site (K(d) = 10 microM) was much weaker than that of the third and fourth sites (K(d) = 0.4 microM). Myr-Frq1 bound Ca(2+) with a K(d)app of 3 microM and a positive Hill coefficient (n = 1.25), suggesting that the N-myristoyl group confers some degree of cooperativity in Ca(2+) binding, as seen previously in recoverin. Both the NMR and fluorescence spectra of Frq1 exhibited very large Ca(2+)-dependent differences, indicating major conformational changes induced upon Ca(2+) binding. Nearly complete sequence-specific NMR assignments were obtained for the entire carboxy-terminal domain (residues K100-I190). Assignments were made for 20% of the residues in the amino-terminal domain; unassigned residues exhibited very broad NMR signals, most likely due to Frq1 dimerization. NMR chemical shifts and nuclear Overhauser effect (NOE) patterns of Ca(2+)-bound Frq1 were very similar to those of Ca(2+)-bound recoverin, suggesting that the overall structure of Frq1 resembles that of recoverin. A model of the three-dimensional structure of Ca(2+)-bound Frq1 is presented based on the NMR data and homology to recoverin. N-myristoylation of Frq1 had little or no effect on its NMR and fluorescence spectra, suggesting that the myristoyl moiety does not significantly alter Frq1 structure. Correspondingly, the NMR chemical shifts for the myristoyl group in both Ca(2+)-free and Ca(2+)-bound myr-Frq1 were nearly identical to those of free myristate in solution, indicating that the fatty acyl chain is solvent-exposed and not sequestered within the hydrophobic core of the protein, unlike the myristoyl group in Ca(2+)-free recoverin. Subcellular fractionation experiments showed that both the N-myristoyl group and Ca(2+)-binding contribute to the ability of Frq1 to associate with membranes.

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The 545-residue Cln2 protein, like the other G1 cyclins of Saccharomyces cerevisiae, is a very unstable protein. This instability is thought to play a critical role in regulating cell cycle progression. The carboxyl-terminal domains of Cln2 and the other G1 cyclins contain sequences rich in Pro, Glu (and Asp), Ser, and Thr (so-called PEST motifs) that have been postulated to make up the signals that are responsible for the rapid degradation of these and other unstable proteins. To test this hypothesis, the carboxyl-terminal 178 residues of Cln2 were fused to the C terminus of a reporter enzyme, a truncated form of human thymidine kinase (hTK delta 40). The resulting chimeric protein (hTK delta 40-Cln2) retained thymidine kinase activity but was markedly less stable than hTK, hTK delta 40, or an hTK-beta-galactosidase fusion protein, as judged by enzyme assay, immunoblotting with anti-hTK antibodies, pulse-chase analysis of the radiolabeled polypeptides, and ability to support the growth of a thymidylate auxotroph (cdc21 mutant) on thymidine-containing medium. Thus, the presence of the Cln2 PEST domain was sufficient to destabilize a heterologous protein. Furthermore, the half-life of hTK delta 40-Cln2 was similar to that of authentic Cln2, and the rate of degradation of neither protein was detectably enhanced by treatments known to cause G1 arrest, including exposure of MATa haploids to alpha-factor mating pheromone and shifting cdc28ts and cdc34ts mutants to the restrictive temperature. These results suggest that the major signals responsible for Cln2 instability are confined to its C-terminal third. Because hTK delta 40-Cln2 and Cln2 were expressed from heterologous promoters yet their half-lives both in asynchronous cultures and when arrested at various cell cycle stages were always similar, the Cln2 PEST domain contains a signal for rapid protein turnover that is constitutively active and operative throughout the cell cycle. Removal of the 37 codons that encode the most prominent PEST-like segment from either hTK delta 40-Cln2 or Cln2 decreased the turnover rate of the resulting proteins, as expected; however, an hTK delta 40 chimera containing only this 37-residue segment was not detectably destabilized, suggesting that this PEST sequence, when removed from its normal context, is not a self-contained determinant of protein instability.

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