cdc25C induces mitosis by activating the cdc2-cyclin B complex. The intracellular localization of cyclin B1 is regulated in a cell cycle-specific manner, and its entry into the nucleus may be required for the initiation of mitosis. To determine the cellular localization of cdc25C, monoclonal antibodies specific for cdc25C were developed and used to demonstrate that in human cells, cdc25C is retained in the cytoplasm during interphase. A deletion analysis identified a 58-amino-acid region (amino acids 201 to 258) in cdc25C that was required for the cytoplasmic localization of cdc25C. This region contained a specific binding site for 14-3-3 proteins, and mutations in cdc25C that disrupted 14-3-3 binding also disrupted the cytoplasmic localization of cdc25C during interphase. cdc25C proteins that do not contain a binding site for 14-3-3 proteins showed a pancellular localization and an increased ability to induce premature chromosome condensation. The cytoplasmic localization of cdc25C was not altered by ␥ irradiation or treatment with the nuclear export inhibitor leptomycin B. These results suggest that 14-3-3 proteins may negatively regulate cdc25C function by sequestering cdc25C in the cytoplasm.In eukaryotic cells, an active cyclin-dependent kinase complex, cdc2-cyclin B1, promotes entry into mitosis. Prior to mitosis, kinase activity is inhibited by phosphorylation of the cdc2 catalytic subunit on two residues, threonine 14 (T14) and tyrosine 15 (Y15) (reviewed in reference 46). Several kinases, including wee1 (3, 21, 48), mik1 (32), and myt1 (36, 42), phosphorylate cdc2 at residues T14 and Y15 and inhibit mitotic progression. Entry into mitosis is dependent on dephosphorylation of the T14 and Y15 residues, resulting in the formation of an active cdc2-cyclin B complex (reviewed in reference 46). Inhibition of cdc2 dephosphorylation is a target of the DNA replication and DNA damage checkpoints in both yeast and mammalian cells (reviewed in references 45 and 46

Inactivation of the retinoblastoma tumor suppressor protein (pRB) contributes to tumorigenesis in a wide variety of cancers. In contrast, the role of the two pRB-related proteins, p130 and p107, in oncogenic transformation is unclear. The LXCXE domain of simian virus 40 large T antigen (TAg) specifically binds to pRB, p107, and p130. We have previously shown that the N terminus and the LXCXE domain of TAg cooperate to alter the phosphorylation state of p130 and p107. Here, we demonstrate that TAg promotes the degradation of p130 and that the N terminus of TAg is required for this activity. The N terminus of TAg has homology to the J domain of the DnaJ family of molecular chaperone proteins. Mutants with mutations in the J-domain homology region of TAg are defective for altering p130 and p107 phosphorylation and for p130 degradation. A heterologous J-domain from a human DnaJ protein can functionally substitute for the N terminus of TAg in the effect on p107 and p130 phosphorylation and p130 stability. We further demonstrate that the J-domain homology region of TAg confers a growth advantage to wild-type mouse embryo fibroblasts (MEFs) but is dispensable in the case of MEFs lacking both p130 and p107. This indicates that p107 and p130 have overlapping growth-suppressing activities whose inactivation is mediated by the J domain of TAg.The retinoblastoma tumor suppressor gene (Rb-1) is mutated in all cases of retinoblastoma and is also frequently mutated in a variety of other cancers (55,91,92). Loss of the antiproliferative activity of the RB protein (pRB) is thought to play a critical role in oncogenic transformation. There are two other members of the RB family of proteins, p107 and p130. These proteins have a high degree of sequence homology to pRB (31,38,56,62). pRB, p107, and p130 share a number of functional properties, including the ability to associate with and negatively regulate members of the E2F family of transcription factors (3,6,8,36,42,78,89) and the ability to induce a G 1 arrest in certain sensitive cell types (16,43,103). The presence of a p130-E2F complex has been proposed to define a quiescent, or G 0 , state of a cell (80). pRB, p107, and p130 are phosphorylated in a cell cycle-dependent manner (2,4,10,21,61,96). Phosphorylation in the mid/late G 1 phase inactivates the growth-suppressive properties of pRB and possibly also of p107 and p130 (15,43,103). pRB, p107, and p130 associate with cyclins (28,30,32,38,47,54,56,78) and are likely to be phosphorylated by cyclin-dependent kinases (cdks). Despite all these similarities, no mutations in p107 or p130 have yet been described in human tumors, and the role of these two proteins in oncogenic transformation remains unclear. pRB appears to be an essential protein in mouse embryonic development, as mice with homozygous deletions of the Rb-1 gene die before birth (14, 46, 52). Moreover, mice heterozygous for Rb-1 develop pituitary tumors with a penetrance of almost 100% (40,45,46). In contrast, mice lacking either p107 or p130 apparently develop norm...

Inositol-requiring enzyme 1 (IRE1) is the most highly conserved signaling node of the unfolded protein response (UPR) and represents a potential therapeutic target for a number of diseases associated with endoplasmic reticulum stress. IRE1 activates the XBP-1 transcription factor by site-specific cleavage of two hairpin loops within its mRNA to facilitate its nonconventional splicing and alternative translation. We screened for inhibitors using a construct containing the unique cytosolic kinase and endoribonuclease domains of human IRE1α (hIRE1α-cyto) and a mini-XBP-1 stem-loop RNA as the substrate. One class compounds was salicylaldehyde analogs from the hydrolyzed product of salicylaldimines in the library. Salicylaldehyde analogs were active in inhibiting the site-specific cleavage of several mini-XBP-1 stem-loop RNAs in a dose-dependent manner. Salicyaldehyde analogs were also active in inhibiting yeast Ire1 but had little activity inhibiting RNase L or the unrelated RNases A and T1. Kinetic analysis revealed that one potent salicylaldehyde analog, 3-ethoxy-5,6-dibromosalicylaldehyde, is a non-competitive inhibitor with respect to the XBP-1 RNA substrate. Surface plasmon resonance studies confirmed this compound bound to IRE1 in a specific, reversible and dose-dependent manner. Salicylaldehydes inhibited XBP-1 splicing induced pharmacologically in human cells. These compounds also blocked transcriptional up-regulation of known XBP-1 targets as well as mRNAs targeted for degradation by IRE1. Finally, the salicylaldehyde analog 3-methoxy-6-bromosalicylaldehyde strongly inhibited XBP-1 splicing in an in vivo model of acute endoplasmic reticulum stress. To our knowledge, salicylaldehyde analogs are the first reported specific IRE1 endoribonuclease inhibitors.