Overlapping signals for protein degradation and nuclear localization define a role for intrinsic RAG-2 nuclear uptake in dividing cells

Expression of the recombinase proteins RAG-1 and RAG-2 is discordant: while RAG-1 is relatively long lived, RAG-2 is degraded periodically at the G(1)-S transition. Destruction of RAG-2 is mediated by a conserved interval in the recombination-dispensable region. The need for RAG-2 to reaccumulate in the nucleus at each cell division suggested the existence of an intrinsic RAG-2 nuclear localization signal (NLS). RAG-1 or RAG-2, expressed individually, is a nuclear protein. A screen for proteins that bind the recombination-dispensable region of RAG-2 identified the nuclear transport protein Importin 5. Mutation of residues 499 to 508 in RAG-2 abolished Importin 5 binding, nuclear accumulation, and periodic degradation of RAG-2. The Importin 5 binding site overlaps an NLS, defined by mutagenesis. RAG-1 rescued the localization of degradation-defective, RAG-2 NLS mutants; this required an intact RAG-1 NLS. Mutations in RAG-2 that abolish intrinsic nuclear accumulation but spare periodic degradation impaired recombination in cycling cells; induction of quiescence restored recombination to wild-type levels. Recombination defects were correlated with a cell cycle-dependent defect in the ability of RAG-1 to rescue localization of the RAG-2 mutants. These results suggest that the intrinsic RAG-2 NLS functions in the nuclear uptake of RAG-2 following its reexpression in cycling cells.

cGMP-dependent protein kinase I beta physically and functionally interacts with the transcriptional regulator TFII-I

Transcriptional regulation of the fos promoter by nitric oxide and cGMP can occur by nuclear translocation of cGMP-dependent protein kinase I (G-kinase I) (Gudi, T., Lohmann, S. M., and Pilz, R. B. (1997) Mol. Cell. Biol. 17, 5244-5254). To identify nuclear targets of G-kinase I, we performed a yeast two-hybrid screen with G-kinase I beta as bait. We found that G-kinase I beta interacted specifically with TFII-I, an unusual transcriptional regulator that associates with multiple proteins to modulate both basal and signal-induced transcription. By using purified recombinant proteins, the interaction was mapped to the N-terminal 93 amino acids of G-kinase I beta and one of six 95-amino acid repeats found in TFII-I. In baby hamster kidney cells, cGMP analogs enhanced co-immunoprecipitation of G-kinase I beta and TFII-I by inducing co-localization of both proteins in the nucleus, but in other cell types containing cytoplasmic TFII-I the G-kinase-TFII-I interaction was largely cGMP-independent. G-kinase phosphorylated TFII-I in vitro and in vivo on Ser(371) and Ser(743) outside of the interaction domain. G-kinase strongly enhanced TFII-I transactivation of a serum-response element-containing promoter in COS7 cells, and this effect was lost when Ser(371) and Ser(743) of TFII-I were mutated. TFII-I by itself had little effect on a full-length fos promoter in baby hamster kidney cells, but it synergistically enhanced transcriptional activation by G-kinase I beta. Binding of G-kinase to TFII-I may position the kinase to phosphorylate and regulate TFII-I and/or factors that interact with TFII-I at the serum-response element.

Differential effects of B cell receptor and B cell receptor-FcgammaRIIB1 engagement on docking of Csk to GTPase-activating protein (GAP)-associated p62

The stimulatory and inhibitory pathways initiated by engagement of stimulatory receptors such as the B cell receptor for antigen (BCR) and inhibitory receptors such as Fcgamma receptors of the IIB1 type (FcgammaRIIB1) intersect in ways that are poorly understood at the molecular level. Because the tyrosine kinase Csk is a potential negative regulator of lymphocyte activation, we examined the effects of BCR and FcgammaRIIB1 engagement on the binding of Csk to phosphotyrosine-containing proteins. Stimulation of a B lymphoma cell line, A20, with intact anti-IgG antibody induced a direct, SH2-mediated association between Csk and a 62-kD phosphotyrosine-containing protein that was identified as RasGTPase-activating protein-associated p62 (GAP-A.p62). In contrast, stimulation of A20 cells with anti-IgG F(ab')2 resulted in little increase in the association of Csk with GAP-A.p62. The effect of FcgammaRIIB1 engagement on this association was abolished by blockade of FcgammaRIIB1 with the monoclonal antibody 2.4G2. Furthermore, the increased association between Csk and GAP-A.p62 seen upon stimulation with intact anti-Ig was abrogated in the FcgammaRIIB1-deficient cell line IIA1.6 and recovered when FcgammaRIIB1 expression was restored by transfection. The differential effects of BCR and BCR-FcgammaRIIB1-mediated signaling on the phosphorylation of GAP-A.p62 and its association with Csk suggest that docking of Csk to GAP-A.p62 may function in the negative regulation of antigen receptor-mediated signals in B cells.

Inositol lipid signalling occurs in brush-border membranes during initiation of compensatory renal growth in the rat

Using highly specific mass assays, concentrations of inositol lipids and 1,2-diacylglycerol (DAG) were determined in plasma membranes isolated from rat kidney cortex. Significantly higher concentrations of inositol lipids were determined in brush-border (BBM) than in basal-lateral (BLM) plasma membranes, although DAG concentrations were similar in both. After unilateral nephrectomy, a decrease in PtdIns(4,5)P2 and PtdIns4P, with a concomitant increase in DAG and translocation of protein kinase C (PKC), were observed in BBM but not in BLM isolated from the remaining kidney. On the other hand, stimulation of renal cortical slices with insulin-like growth factor II (IGF-II) or phenylephrine caused similar effects in BLM but not in BBM. Stimulation of phospholipase C activity with translocation of PKC only to BBM in one kidney was also induced by occlusion of blood flow through the contralateral kidney for 15 min. At 30 min after the occlusion was removed and reflow established, DAG concentration and the amount of PKC in BBM returned to control values. These results suggest that an early signal after unilateral nephrectomy is transmitted to cells through BBM and can be switched on and off by blood occlusion and reflow through the contralateral kidney, while hormonal signals caused by IGF-II and phenylephrine are transmitted to cells through BLM.