The C. elegans F-box/WD-repeat protein LIN-23 functions to limit cell division during development

In multicellular eukaryotes, a complex program of developmental signals regulates cell growth and division by controlling the synthesis, activation and degradation of G(1) cell cycle regulators. Here we describe the lin-23 gene of Caenorhabditis elegans, which is required to restrain cell proliferation in response to developmental cues. In lin-23 null mutants, all postembryonic blast cells undergo extra divisions, creating supernumerary cells that can differentiate and function normally. In contrast to the inability to regulate the extent of blast cell division in lin-23 mutants, the timing of initial cell cycle entry of blast cells is not affected. lin-23 encodes an F-box/WD-repeat protein that is orthologous to the Saccharomyces cerevisiae gene MET30, the Drosophila melanogaster gene slmb and the human gene betaTRCP, all of which function as components of SCF ubiquitin-ligase complexes. Loss of function of the Drosophila slmb gene causes the growth of ectopic appendages in a non-cell autonomous manner. In contrast, lin-23 functions cell autonomously to negatively regulate cell cycle progression, thereby allowing cell cycle exit in response to developmental signals.

Evolution of cyclin-dependent kinases (CDKs) and CDK-activating kinases (CAKs): differential conservation of CAKs in yeast and metazoa

Cyclin-dependent kinases (CDKs) function as central regulators of both the cell cycle and transcription. CDK activation depends on phosphorylation by a CDK-activating kinase (CAK). Different CAKs have been identified in budding yeast, fission yeast, and metazoans. All known CAKs belong to the extended CDK family. The sole budding yeast CAK, CAK1, and one of the two CAKs in fission yeast, csk1, have diverged considerably from other CDKs. Cell cycle regulatory components have been largely conserved in eukaryotes; however, orthologs of neither CAK1 nor csk1 have been identified in other species to date. To determine the evolutionary relationships of yeast and metazoan CAKs, we performed a phylogenetic analysis of the extended CDK family in budding yeast, fission yeast, humans, the fruit fly Drosophila melanogaster, and the nematode Caenorhabditis elegans. We observed that there were 10 clades for CDK-related genes, of which seven appeared ancestral, containing both yeast and metazoan genes. The four clades that contain CDKs that regulate transcription by phosphorylating the carboxyl-terminal domain (CTD) of RNA Polymerase II generally have only a single orthologous gene in each species of yeast and metazoans. In contrast, the ancestral cell cycle CDK (analogous to budding yeast CDC28) gave rise to a number of genes in metazoans, as did the ancestor of budding yeast PHO85. One ancestral clade is unique in that there are fission yeast and metazoan members, but there is no budding yeast ortholog, suggesting that it was lost subsequent to evolutionary divergence. Interestingly, CAK1 and csk1 branch together with high bootstrap support values. We used both the relative apparent synapomorphy analysis (RASA) method in combination with the S-F method of sampling reduced character sets and gamma-corrected distance methods to confirm that the CAK1/csk1 association was not an artifact of long-branch attraction. This result suggests that CAK1 and csk1 are orthologs and that a central aspect of CAK regulation has been conserved in budding and fission yeast. Although there are metazoan CDK-family members for which we could not define ancestral lineage, our analysis failed to identify metazoan CAK1/csk1 orthologs, suggesting that if the CAK1/csk1 gene existed in the metazoan ancestor, it has not been conserved.

The F-box protein family

The F-box is a protein motif of approximately 50 amino acids that functions as a site of protein-protein interaction. F-box proteins were first characterized as components of SCF ubiquitin-ligase complexes (named after their main components, Skp I, Cullin, and an F-box protein), in which they bind substrates for ubiquitin-mediated proteolysis. The F-box motif links the F-box protein to other components of the SCF complex by binding the core SCF component Skp I. F-box proteins have more recently been discovered to function in non-SCF protein complexes in a variety of cellular functions. There are 11 F-box proteins in budding yeast, 326 predicted in Caenorhabditis elegans, 22 in Drosophila, and at least 38 in humans. F-box proteins often include additional carboxy-terminal motifs capable of protein-protein interaction; the most common secondary motifs in yeast and human F-box proteins are WD repeats and leucine-rich repeats, both of which have been found to bind phosphorylated substrates to the SCF complex. The majority of F-box proteins have other associated motifs, and the functions of most of these proteins have not yet been defined.

CUL-2 is required for the G1-to-S-phase transition and mitotic chromosome condensation in Caenorhabditis elegans

The human cullin protein CUL-2 functions in a ubiquitin-ligase complex with the von Hippel-Lindau (VHL) tumour suppressor protein. Here we show that, in Caenorhabditis elegans, cul-2 is expressed in proliferating cells and is required at two distinct points in the cell cycle, the G1-to-S-phase transition and mitosis. cul-2 mutant germ cells undergo a G1-phase arrest that correlates with accumulation of CKI-1, a member of the CIP/KIP family of cyclin-dependent-kinase inhibitors. In cul-2 mutant embryos, mitotic chromosomes are unable to condense, leading to unequal DNA segregation, chromosome bridging and the formation of multiple nuclei.

Loss of Cul1 results in early embryonic lethality and dysregulation of cyclin E

The sequential timing of cell-cycle transitions is primarily governed by the availability and activity of key cell-cycle proteins. Recent studies in yeast have identified a class of ubiquitin ligases (E3 enzymes) called SCF complexes, which regulate the abundance of proteins that promote and inhibit cell-cycle progression at the G1-S phase transition. SCF complexes consist of three invariable components, Skp1, Cul-1 (Cdc53 in yeast) and Rbx1, and a variable F-box protein that recruits a specific cellular protein to the ubquitin pathway for degradation. To study the role of Cul-1 in mammalian development and cell-cycle regulation, we generated mice deficient for Cul1 and analysed null embryos and heterozygous cell lines. We show that Cul1 is required for early mouse development and that Cul1 mutants fail to regulate the abundance of the G1 cyclin, cyclin E (encoded by Ccne), during embryogenesis.

Human CUL1 forms an evolutionarily conserved ubiquitin ligase complex (SCF) with SKP1 and an F-box protein

The SCF ubiquitin ligase complex of budding yeast triggers DNA replication by catalyzing ubiquitination of the S phase cyclin-dependent kinase inhibitor SIC1. SCF is composed of three proteins-ySKP1, CDC53 (Cullin), and the F-box protein CDC4-that are conserved from yeast to humans. As part of an effort to identify components and substrates of a putative human SCF complex, we isolated hSKP1 in a two-hybrid screen with hCUL1, the closest human homologue of CDC53. Here, we show that hCUL1 associates with hSKP1 in vivo and directly interacts with both hSKP1 and the human F-box protein SKP2 in vitro, forming an SCF-like particle. Moreover, hCUL1 complements the growth defect of yeast cdc53(ts) mutants, associates with ubiquitination-promoting activity in human cell extracts, and can assemble into functional, chimeric ubiquitin ligase complexes with yeast SCF components. Taken together, these data suggest that hCUL1 functions as part of an SCF ubiquitin ligase complex in human cells. Further application of biochemical assays similar to those described here can now be used to identify regulators/components of hCUL1-based SCF complexes, to determine whether the hCUL2-hCUL5 proteins also are components of ubiquitin ligase complexes in human cells, and to screen for chemical compounds that modulate the activities of the hSKP1 and hCUL1 proteins.

cul-1 is required for cell cycle exit in C. elegans and identifies a novel gene family

The gene cul-1 (formerly lin-19) is a negative regulator of the cell cycle in C. elegans. Null mutations cause hyperplasia of all tissues. cul-1 is required for developmentally programmed transitions from the G1 phase of the cell cycle to the GO phase or the apoptotic pathway. Moreover, the mutant phenotype suggests that G1-to-S phase progression is accelerated, overriding mechanisms for mitotic arrest and producing abnormally small cells. Significantly, diverse aspects of cell fate and differentiation are unaffected in cul-1 mutants. cul-1 represents a conserved family of genes, designated cullins, with at least five members in nematodes, six in humans, and three in budding yeast.

Oncogenic v-Abl tyrosine kinase can inhibit or stimulate growth, depending on the cell context

The v-abl oncogene of Abelson murine leukemia virus (A-MuLV) induces two opposite phenotypes in NIH3T3 cells. In the majority of cells, v-abl causes a growth arrest at the G1 phase of the cell cycle; while in a minority of cells, v-abl abrogates the requirement for growth factors. Using temperature sensitive mutants, it can be demonstrated that v-Abl tyrosine kinase is required for growth inhibition or stimulation. The two phenotypes are not caused by mutations or differences in the expression of v-Abl, but are dependent on the cell context. Two stable subclones of NIH3T3 cells have been isolated that exhibit similar morphology and growth characteristics. However, upon infection with A-MuLV, the 'positive' cells become serum- and anchorage-independent, whereas the 'negative' cells become arrested in G1. The positive phenotype is dominant, shown by cell fusion, and treatment with 5-azacytidine converts the negative cells to the positive phenotype. Activation of v-Abl tyrosine kinase induces the serum-responsive genes in the positive but not in the negative cells. Transactivation of the c-fos promoter by v-Abl in transient assays is also restricted to the positive cells. These results show that v-Abl tyrosine kinase is not an obligatory activator of growth, but requires a permissive cellular context to manifest its mitogenic function.

Cell cycle-regulated binding of c-Abl tyrosine kinase to DNA

The proto-oncogene c-abl encodes a protein tyrosine kinase that is localized in the cytoplasm and the nucleus. The large carboxyl-terminal segment of c-Abl was found to contain a DNA-binding domain that was necessary for the association of c-Abl with chromatin. The DNA-binding activity of c-Abl was lost during mitosis when the carboxyl-terminal segment became phosphorylated. In vitro phosphorylation of the DNA-binding domain by cdc2 kinase abolished DNA binding. Homozygous mutant mice expressing a c-Abl tyrosine kinase without the DNA-binding domain have been reported to die of multiple defects at birth. Thus, binding of the c-Abl tyrosine kinase to DNA may be essential to its biological function.

Differential phosphorylation of c-Abl in cell cycle determined by cdc2 kinase and phosphatase activity

The product of the c-abl proto-oncogene (c-Abl) is phosphorylated on three sites during interphase and seven additional sites during mitosis. Two interphase and all mitotic c-Abl sites are phosphorylated by cdc2 kinase isolated from either interphase or mitotic cells, with the mitotic cdc2 having an 11-fold higher activity. Inhibition of phosphatases with okadaic acid in interphase cells leads to the phosphorylation of c-Abl mitotic sites, indicating that those sites are preferentially dephosphorylated during interphase. The differential phosphorylation of c-Abl in the cell cycle is therefore determined by an equilibrium between cdc2 kinase and protein phosphatase activities. Treatment of interphase cells with okadaic acid leads to a rounded morphology similar to that observed during mitosis.

Reversible dependence on growth factor interleukin-3 in myeloid cells expressing temperature sensitive v-abl oncogene

Abelson murine leukemia virus (A-MuLV) has been shown to abrogate the requirement for growth factor interleukin-3 (IL-3) in a variety of hematopoietic cell lineages by a non-autocrine mechanism. By infecting an IL-3 dependent myeloid cell line, FDC-P1, with A-MuLV containing temperature sensitive tyrosine kinase mutants of the v-abl oncogene, cell lines with temperature sensitive IL-3 independence phenotype were established. At the permissive temperature, cells expressing the ts oncogenes contained 20 fold higher levels of tyrosine-phosphorylated proteins than uninfected cells and were completely IL-3 independent. When shifted to the restrictive temperature, ts A-MuLV infected cells still contained 5 to 10 fold higher levels of phosphotyrosine but became dependent on IL-3 for growth. These results demonstrate that the maintenance of A-MuLV induced IL-3 independence requires the continuous function of the v-abl oncogene.

Isolation of temperature-sensitive tyrosine kinase mutants of v-abl oncogene by screening with antibodies for phosphotyrosine

Temperature-sensitive protein-tyrosine kinase (EC 2.7.1.112) mutants of the oncogene v-abl have been obtained by a direct screening of kinase mutants in bacteria. The v-abl oncogene was expressed in Escherichia coli as a trpE/v-abl fusion protein from the trp promoter. The expression plasmid was mutagenized in vitro and then transfected into E. coli. Bacteria that produced defective tyrosine kinases were distinguished from those producing wild-type v-abl kinases by hybridization with antibodies specific for phosphotyrosine. Two independent mutations that generated temperature-sensitive tyrosine kinases were found to be located in a 12-amino acid region in the tyrosine kinase domain of the v-abl-encoded protein. These mutant v-abl oncogenes displayed temperature-sensitive transforming activity when expressed in NIH 3T3 cells. Cells transformed by these temperature-sensitive tyrosine kinase mutants could be shifted between the transformed and untransformed states by changing their growth temperature. These results confirmed the crucial role of tyrosine kinase activity in the v-abl-mediated oncogenesis.