CD8 deficiency is an autosomal recessive form of severe combined immunodeficiency diseases characterized by the absence of CD8؉ T lymphocytes and impaired T cell functions. We identified two novel missense mutations in the zap70 genes of a CD8-deficiency patient. One mutation (P80Q) affects a residue in an SH2 domain and another (M572L) in the kinase subdomain XI. Both mutations cause a degradation of ZAP70 protein in a temperature-sensitive manner through an ATPdependent and proteasome-independent pathway. We further demonstrated that Cdc37, a protein kinase-specific chaperone, bound to M572L but not P80Q mutant and restored the expression of the M572L mutant when overexpressed. The restoration of M572L mutant by Cdc37 required the function of HSP90. These results indicate that Cdc37 in conjunction with HSP90 functions as a molecular chaperone for a temperature-sensitive kinase domain mutant of ZAP70.CD8 deficiency is an autosomal recessive form of severe combined immunodeficiency diseases (SCIDs) 1 and is associated with defects in the ZAP70 protein tyrosine kinase (PTK), which plays a pivotal role in signal transduction through the T cell receptor (TCR) (1-5). Upon TCR stimulation, ZAP70 is recruited to tyrosine-phosphorylated immunoreceptor tyrosinebased activation motifs (ITAMs) within the cytoplasmic domains of TCR subunits; this is an essential step for ZAP70 activation and subsequent cellular signaling pathways (6 -8). Association of ZAP70 with the TCR is mediated by an interaction between the two SH2 domains of the ZAP70 molecule arranged in tandem and the two phosphorylated tyrosine residues in the ITAM (9 -13).All of the mutations in the ZAP70 molecule that have been reported to cause SCID in humans cluster around the kinase subdomain VIII, resulting in instability of the ZAP70 protein (3-5). In general, misfolded proteins are recognized by cellular proteins, chaperones and proteases. Molecular chaperones assist in the proper folding of misfolded polypeptide and render it functional, whereas proteases eliminate such a polypeptide (14). It is thus likely that the counterbalance between chaperones and proteases determines the stability of mutant ZAP70 molecules. With respect to proteases, it is widely accepted that the ubiquitin-proteasome system is involved in degradation of abnormal proteins (15,16). However, the contribution of proteasomes to the instability of mutant ZAP70 proteins is yet to be determined.We show here that two novel mis-sense mutations in the zap70 genes of a CD8-deficiency patient cause degradation of ZAP70 protein in vivo in a temperature-sensitive (ts) manner through an ATP-dependent and proteasome-independent pathway. We also show that overexpression of Cdc37 (17, 18), a protein kinase-specific molecular chaperone, preferentially restores the expression of a kinase-domain mutant of the ZAP70 molecule even at the nonpermissive temperature.

Mitogen-activated protein kinase (MAPK) cascades are involved in a variety of cellular responses including proliferation, differentiation, and apoptosis. We have developed an expression screening method to detect in vivo substrates of MAPKs in mammalian cells, and identified a membrane protein, linker for activation of T cells (LAT), as an MAPK target. LAT, an adapter protein essential for Tcell signaling, is phosphorylated at its Thr 155 by ERK in response to T-cell receptor stimulation. Thr 155 phosphorylation reduces the ability of LAT to recruit PLCc1 and SLP76, leading to attenuation of subsequent downstream events such as [Ca 2 þ ] i mobilization and activation of the ERK pathway. Our data reveal a new role for MAPKs in a negative feedback loop in T-cell activation via threonine phosphorylation of LAT.

Class IA and IB phosphoinositide 3-kinases (PI3Ks) have been shown to regulate mast cell functions such as proliferation, development, survival and degranulation, but the functional redundancy between these two PI3K signaling pathways in mast cells remains unclear. Here, we have generated mice deficient in both class IA regulatory subunit p85α and class IB catalytic subunit p110γ, and show that p85α(-/-)p110γ(-/-) mice exhibit a more severe defect in mast cell development than single-knockout mice. In addition, the in vivo passive cutaneous anaphylaxis reaction of p85α(-/-)p110γ(-/-) mice was nearly completely abrogated, whereas single-knockout mice exhibit just marginal reduction. Pharmacological inactivation of Akt in wild-type bone marrow-derived mast cells (BMMCs) led to partial reduction of degranulation, while over-expression of a constitutively active Akt partially restored the impaired degranulation in p85α(-/-)p110γ(-/-) BMMCs. We also found that the extracellular signal-regulated kinase (ERK) signaling pathway was activated in a PI3K-dependent manner upon FcεRI stimulation and that simultaneous inhibition of Akt and ERK resulted in nearly complete blockade of FcεRI-induced degranulation. Our data provide evidence that Akt and ERK pathways play redundant roles in FcεRI-induced degranulation.

Plasmids expressing Gl and G2 cyclins were introduced into the Saos-2 cell system monitoring p53-mediated transactivation [(1993) Oncogene 8, 5431. Cyclin E, but not other cyclins, enhanced the p53-mediated transactivation about 2-fold. Co-transfectton of a CDK2 expression plasmid caused a 30% increase in the extent of the pS3-mediated transactivation. Moreover, the transfected ~53 protein became phosphorylated coordmately with the enhanced transactivation.The close correlation between transactivatton and p53 phosphorylatton suggests that phosphorylation is mvolved in positive regulation for the transactivatton by ~53.