We determined composition and relative roles of deubiquitylating proteins associated with the 26S proteasome in mammalian cells. Three deubiquitylating activities were associated with the 26S proteasome: two from constituent subunits, Rpn11/S13 and Uch37, and one from a reversibly associated protein, Usp14. RNA interference (RNAi) of Rpn11/S13 inhibited cell growth, decreased cellular proteasome activity via disrupted 26S proteasome assembly, and inhibited cellular protein degradation. In contrast, RNAi of Uch37 or Usp14 had no detectable effect on cell growth, proteasome structure or proteolytic capacity, but accelerated cellular protein degradation. RNAi of both Uch37 and Usp14 also had no effect on proteasome structure or proteolytic capacity, but inhibited cellular protein degradation. Thus, proper proteasomal processing of ubiquitylated substrates requires Rpn11 plus either Uch37 or Usp14. Although the latter proteins feature redundant deubiquitylation functions, they also appear to exert noncatalyic effects on proteasome activity that are similar to but independent of one another. These results reveal unexpected functional relationships among multiple deubiquitylating proteins and suggest a model for mammalian 26S proteasome function whereby their concerted action governs proteasome function by linking deubiquitylation to substrate hydrolysis.

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Cerebellar granule neurons undergo apoptosis when switched from medium containing depolarizing levels of potassium (high K + medium, HK) to medium containing low K + (LK). NF-jB, a ubiquitously expressed transcription factor, is involved in the survival-promoting effects of HK. However, neither the expression nor the intracellular localization of the five NF-jB proteins, or of IjB-a and IjB-b, are altered in neurons primed to undergo apoptosis by LK, suggesting that uncommon mechanisms regulate NF-jB activity in granule neurons. In this study, we show that p65 interacts with the transcriptional co-activator, CREB-binding protein (CBP), in healthy neurons. The decrease in NF-jB transcriptional activity caused by LK treatment is accompanied by a reduction in the interaction between p65 and CBP, an alteration that is accompanied by hyperphosporylation of CBP. LK-induced CBP hyperphosphorylation can be mimicked by inhibitors of protein phosphatase (PP) 2A and PP2A-like phosphatases such as okadaic acid and cantharidin, which also causes a reduction in p65-CBP association. In addition, treatment with these inhibitors induces cell death. Treatment with high concentrations of the broad-spectrum kinase inhibitor staurosporine prevents LK-mediated CBP hyperphosphorylation and inhibits cell death. In vitro kinase assays using glutathione-Stransferase (GST)-CBP fusion proteins map the LK-regulated site of phosphorylation to a region spanning residues 1662-1840 of CBP. Our results are consistent with possibility that LK-induced apoptosis is triggered by CBP hyperphosphorylation, an alteration that causes the dissociation of CBP and NF-jB.

The Ly49 family of genes encode NK cell receptors that bind class I MHC Ags and transmit negative signals if the cytoplasmic domains have immunoregulatory tyrosine-based inhibitory motifs (ITIMs). 5E6 mAbs recognize Ly49C and Ly49I receptors and depletion of 5E6+ NK cells prevents rejection of allogeneic or parental-strain H2d bone marrow cell (BMC) grafts. To determine the function of the Ly49I gene in the rejection of BMC grafts, we transfected fertilized eggs of FVB mice with a vector containing DNA for B6 strain Ly49I (Ly49IB6). Ly49IB6 is ITIM+ and is recognized by 5E6 as well as Ly49I-specific 8H7 mAbs. Normal FVB H2q mice reject H2b but not H2d BMC allografts, and the rejection of H2b BMC was inhibited partially by anti-NK1.1 and completely by anti-asialo GM1, but not by anti-CD8, Abs. In FVB mice, NK1.1 is expressed on only 60% NK cells. FVB.Ly49IB6 hosts failed to reject H2d or H2b BMC, but did reject class I-deficient TAP-1−/− BMC, indicating that NK cells were functional. Nondepleting doses of anti-Ly49I Abs reversed the acceptance of H2b BMC by FVB.Ly49IB6 mice. FVB.Ly49IB6+/− mice were crossed and back-crossed with 129 mice—H2b, 5E6−, poor responders to H2d BMC grafts. While transgene-negative H2b/q F1 or first-generation back-crossed mice rejected H2b marrow grafts (hybrid resistance), transgene-positive mice did not. Thus B6 strain Ly49I receptors transmit inhibitory signals from H2b MHC class I molecules. Moreover, Ly49IB6 has no positive influence on the rejection of H2d allografts.

The NF-kB transcription factor consists of dimeric complexes belonging to the Rel family, which include p50, p52, p65 (RelA), RelB and c-Rel. NF-kB activity is tightly controlled by IkB proteins which bind to NF-kB preventing its translocation to the nucleus. Activation of NF-kB is most often mediated by IkB degradation, which permits NF-kB to enter the nucleus. We investigated the role of NF-kB in the survival of cerebellar granule neurons. We found that survival of these neurons in high potassium medium is blocked by three separate inhibitors of NF-kB activity: SN-50, N-tosyl-L-phenylalanine chloromethyl ketone and pyrrolidinedithiocarbamate, indicating that NF-kB is required for neuronal survival. Gel-shift assays reveal three complexes that bind to the NF-kB binding site in high potassium medium. Switching these cultures to low potassium medium, a stimulus that leads to apoptotic death, causes a reduction in the level of the largest complex, which contains p65. Overexpression of p65 by transfection inhibits low potassium-induced apoptosis, whereas overexpression of IkBa promotes apoptosis even in high potassium medium. Surprisingly, however, neither the level of endogenous p65 nor that of IkBa and IkBb is altered by low potassium treatment. Similarly, no changes are seen in the nuclear or cytoplasmic levels of p50, p52, RelB and c-Rel. Phosphorylation of p65, which can lead to its activation, is unchanged. Phosphorylation of IkBb is, however, reduced by low potassium treatment. Besides being necessary for high potassium-mediated neuronal survival, NF-kB is also involved in the survival-promoting effects of IGF-1 and cAMP as judged by the ability of SN-50 to inhibit the actions of these survival factors and the ability of these factors to inhibit the low potassium-induced alterations in the DNA-binding activity of NF-kB. Taken together, our results show that NF-kB may represent a point of convergence in the signaling pathways activated by different survival factors and that uncommon mechanisms might be involved in NF-kB-mediated survival of cerebellar granule neurons.