The F-box protein Skp2 mediates c-Myc ubiquitylation by binding to the MB2 domain. However, the turnover of cMyc is largely dependent on phosphorylation of threonine-58 and serine-62 in MB1, residues that are often mutated in cancer. We now show that the F-box protein Fbw7 interacts with and thereby destabilizes c-Myc in a manner dependent on phosphorylation of MB1. Whereas wild-type Fbw7 promoted c-Myc turnover in cells, an Fbw7 mutant lacking the F-box domain delayed it. Furthermore, depletion of Fbw7 by RNA interference increased both the abundance and transactivation activity of c-Myc. Accumulation of cMyc was also apparent in mouse Fbw7 À/À embryonic stem cells. These observations suggest that two F-box proteins, Fbw7 and Skp2, differentially regulate c-Myc stability by targeting MB1 and MB2, respectively.

Protein kinase C (PKC), which comprises 11 closely related isoforms, has been implicated in a wide variety of cellular processes, such as growth, differentiation, secretion, apoptosis and tumour development. Among the PKC isotypes, PKC-delta is unique in that its overexpression results in inhibition of cell growth. Here we show that mice that lack PKC-delta exhibit expansion of the B-lymphocyte population with the formation of numerous germinal centres in the absence of stimulation. The rate of proliferation in response to stimulation was greater for B cells from PKC-delta-deficient mice than for those from wild-type mice. Adoptive transfer experiments suggested that the hyperproliferation phenotype is B-cell autonomous. Production of interleukin-6 was markedly increased in B cells of PKC-delta-null mice as a result of an increase in the DNA-binding activity of NF-IL6. Furthermore, the PKC-delta-deficient mice contain circulating autoreactive antibodies and display immune-complex-type glomerulonephritis, as well as lymphocyte infiltration in many organs. These results suggest that PKC-delta has an indispensable function in negative regulation of B-cell proliferation, and is particularly important for the establishment of B-cell tolerance.

The abundance of the cyclin-dependent kinase (CDK) inhibitor p57 Kip2 , an important regulator of cell cycle progression, is thought to be controlled by the ubiquitin-proteasome pathway. The Skp1͞ Cul1͞F-box (SCF)-type E3 ubiquitin ligase complex SCF Skp2 has now been shown to be responsible for regulating the cellular level of p57 Kip2 by targeting it for ubiquitylation and proteolysis. The elimination of p57 Kip2 was impaired in Skp2 ؊/؊ cells, resulting in abnormal accumulation of the protein. Coimmunoprecipitation analysis also revealed that Skp2 interacts with p57 Kip2 in vivo. Overexpression of WT Skp2 promoted degradation of p57 Kip2 , whereas expression of a dominant negative mutant of Skp2 prolonged the half-life of p57 Kip2 . Mutation of the threonine residue (Thr-310) of human p57 Kip2 that is conserved between the COOH-terminal QT domains of p57 Kip2 and p27 Kip1 prevented the effect of Skp2 on the stability of p57 Kip2 , suggesting that phosphorylation at this site is required for SCF Skp2 -mediated ubiquitylation. Finally, the purified recombinant SCF Skp2 complex mediated p57 Kip2 ubiquitylation in vitro in a manner dependent on the presence of the cyclin E-CDK2 complex. These observations thus demonstrate that the SCF Skp2 complex plays an important role in cell-cycle progression by determining the abundance of p57 Kip2 and that of the related CDK inhibitor p27 Kip1 .

The multiple C. elegans SKR proteins exhibit marked differences in their association with Cullins and F-box proteins, in tissue specificity of expression, and in phenotypes associated with functional suppression by RNAi. At least eight of the SKR proteins may, like F-box proteins, act as variable components of the SCF complex in C. elegans.