The cullin family of proteins is involved in the ubiquitin-mediated degradation of cell cycle regulators. Relatively little is known about the function of the CUL-4A cullin, but its overexpression in breast cancer suggests CUL-4A might also regulate the cell cycle. In addition, since other cullins are required for normal development, we hypothesized that CUL-4A is involved in regulating cell cycle progression during differentiation. We observed that CUL-4A mRNA and protein levels decline 2.5-fold during the differentiation of PLB-985 myeloid cells into granulocytes. To examine the significance of this observation, we overexpressed CUL-4A in these cells and found that modest (< 2-fold), enforced expression of CUL-4A attenuates terminal granulocytic differentiation and instead promotes proliferation. This overexpression similarly affects the differentiation of these cells into macrophages. We recently reported that nearly one half of CUL-4A ؉/؊ mice are nonviable, and in this report, we show that the viable heterozygous mice, which have reduced CUL-4A expression, have dramatically fewer erythroid and multipotential progenitors than normal controls. Together these results indicate that appropriate CUL-4A expression is essential for embryonic development and for cell cycle regulation during
IntroductionUbiquitin-mediated degradation plays a critical role in controlling the turnover of cell cycle regulators. 1,2 The adenosine triphosphate (ATP)-dependent attachment of ubiquitin to a ubiquitin-activating enzyme (E1) activates ubiquitin for transfer to a ubiquitinconjugating enzyme (E2) and then to a ubiquitin ligase (E3), which transfers ubiquitin to a substrate protein. 3 Repetition of this ubiquitin transferase reaction results in the attachment of a polyubiquitin chain to the substrate, which is then recognized by the 26S proteasome and degraded. Much of the ubiquitin pathway's substrate specificity derives from E3 ligases, so regulating E3 activity is a likely mechanism for controlling ubiquitin-mediated degradation. Cullins are a core component of a subset of E3 ligases (described below). In this report, we examine the function of the CUL-4A cullin and find it plays a role in regulating granulocytic differentiation.Multisubunit RING (Really Interesting New Gene) E3 ligases contain at least 5 subunits: a RING finger protein, an E2, an adaptor protein, a substrate recognition subunit, and a cullin. 3 An extensively studied type of multisubunit RING E3 is SCF (Skp1, Cdc53/cullin, F-box protein), which was initially characterized in Saccharomyces cerevisiae. 1,2 Skp1 (the adaptor protein), Cdc53(the cullin), and Rbx1 (the RING finger protein) make up the SCF core. This core binds an E2 and various F-box proteins (such as Cdc4, Grr1, and Met30), which are responsible for substrate recognition. The resulting SCF complexes (SCF Cdc4 , SCF Grr1 , and SCF Met30 ) target for degradation a variety of cell cycle regulators, including Sic1, Far1, Cdc6, Cln1, Cln2, Gic2, and Swe1.Mammals encode 6 cullins, Cul-1, Cul-2, Cul-3...