Oscillations in the activity of cyclin-dependent kinases (CDKs) promote progression through the eukaryotic cell cycle. This review examines how proteolysis regulates CDK activity—by degrading CDK activators or inhibitors—and also how proteolysis may directly trigger the transition from metaphase to anaphase. Proteolysis during the cell cycle is mediated by two distinct ubiquitin-conjugation pathways. One pathway, requiring CDC34, initiates DNA replication by degrading a CDK inhibitor. The second pathway, involving a large protein complex called the anaphase-promoting complex or cyclosome, initiates chromosome segregation and exit from mitosis by degrading anaphase inhibitors and mitotic cyclins. Proteolysis therefore drives cell cycle progression not only by regulating CDK activity, but by directly influencing chromosome and spindle dynamics.

https://science.sciencemag.org/content/sci/274/5293/1652.full.pdf

How proteolysis drives the cell cycle

Doublecortin is a microtubule-associated protein and is expressed widely by migrating neurons.

https://www.cell.com/cell/pdf/0092-8674(81)90015-5.pdf

Feedback control of mitosis in budding yeast.

The tetratricopeptide repeat (TPR) motif is a protein-protein interaction module found in multiple copies in a number of functionally different proteins that facilitates specific interactions with a partner protein(s). Three-dimensional structural data have shown that a TPR motif contains two antiparallel alpha-helices such that tandem arrays of TPR motifs generate a right-handed helical structure with an amphipathic channel that might accommodate the complementary region of a target protein. Most TPR-containing proteins are associated with multiprotein complexes, and there is extensive evidence indicating that TPR motifs are important to the functioning of chaperone, cell-cycle, transcription, and protein transport complexes. The TPR motif may represent an ancient protein-protein interaction module that has been recruited by different proteins and adapted for specific functions. BioEssays 1999;21:932-939.

The tetratricopeptide repeat: a structural motif mediating protein-protein interactions.

A 20s complex containing CDC27 and CDC16 catalyzes the mitosis-specific conjugation of ubiquitin to cyclin B

During a study of the genetics of nuclear migration in the filamentous fungus Aspergillus nidulans, we cloned a gene, nudF, which is required for nuclear migration during vegetative growth as well as development. The NUDF protein level is controlled by another protein NUDC, and extra copies of the nudF gene can suppress the nudC3 mutation. nudF encodes a protein with 42% sequence identity to the human LIS-1 (Miller-Dieker lissencephaly-1) gene, which is required for proper neuronal migration during brain development. This strong similarity suggests that the LIS-1 gene product may have a function similar to that of NUDF and supports previous findings to suggest that nuclear migration may play a role in neuronal migration.

/pdf/nudf-a-nuclear-migration-gene-in-aspergillus-nidulans-is-4cyxp8sza0.pdf

NudF, a nuclear migration gene in Aspergillus nidulans, is similar to the human LIS-1 gene required for neuronal migration.

A temperature-sensitive mutation in the nudC gene (nudC3) of Aspergillus nidulans specifically prevents the microtubule-based movement of nuclei in this organism at the restrictive temperature. The mutation does not affect short term growth, nuclear division, or the movement of other subcellular organelles. Immunofluorescence analysis of cells blocked at the restrictive temperature, using antitubulin antibodies, shows that the inability of nuclei to move under these conditions is not related to an inability of a particular class of microtubule to form. The inability to move nuclei in this mutant is also shown to be independent of both mitosis and the number of nuclei in the cell as a double mutant carrying both nudC3 and a cell cycle-specific mutation blocks with a single immotile nucleus at the restrictive temperature. The molecular cloning of the nudC gene and sequence analysis reveal that it encodes a previously unidentified protein of 22 kd. Affinity-purified antisera reactive to the nudC protein cross reacts to a single protein of 22 kD in Aspergillus protein extracts. This purified sera failed to reveal a subcellular location for the nudC protein at the level of indirect immunofluorescence. The data presented suggest that the 22-kD nudC gene product functions by interacting between microtubules and nuclei and/or is involved in the generation of force used to move nuclei during interphase.

/pdf/the-molecular-cloning-and-identification-of-a-gene-product-i5kmkn4ygx.pdf

The molecular cloning and identification of a gene product specifically required for nuclear movement in Aspergillus nidulans.

The recessive, temperature-sensitive bimA1 mutation of Aspergillus nidulans blocks nuclei in metaphase at restrictive temperature. To determine whether the bimA product is essential, integrative transformation was used to create a mutation in the bimA gene. The mutation was maintained in a heterokaryon and the phenotype of spores produced by the heterokaryon was analyzed. Molecular disruption of the wild-type bimA gene is recessive in the heterokaryon and causes a metaphase block, demonstrating that bimA is an essential gene for mitosis. bimA was cloned by DNA-mediated complementation of its mutant phenotype at restrictive temperature, and the nucleotide sequence of a full-length cDNA was determined. A single large open reading frame was identified in the cDNA sequence, which predicts a protein containing 806 amino acid residues that is related (30.4% identity) to the Schizosaccharomyces pombe nuc2+ gene product, which also is required for completion of mitosis. The sequence of the bimA gene indicates that it is a member of a family of mostly nuclear proteins that contain a degenerate 34 amino acid repeat, the TPR (tetratricopeptide repeat) gene family.

S A Osmani

Papers

NudF, a nuclear migration gene in Aspergillus nidulans, is similar to the human LIS-1 gene required for neuronal migration.

The molecular cloning and identification of a gene product specifically required for nuclear movement in Aspergillus nidulans.

bimA encodes a member of the tetratricopeptide repeat family of proteins and is required for the completion of mitosis in Aspergillus nidulans.

A negative regulator of mitosis in Aspergillus is a putative membrane-spanning protein.