Cyclin C: The Story of a Non-Cycling Cyclin

Abstract

The class I cyclin family is a well-studied group of structurally conserved proteins that interact with their associated cyclin-dependent kinases (Cdks) to regulate different stages of cell cycle progression depending on their oscillating expression levels. However, the role of class II cyclins, which primarily act as transcription factors and whose expression remains constant throughout the cell cycle, is less well understood. As a classic example of a transcriptional cyclin, cyclin C forms a regulatory sub-complex with its partner kinase Cdk8 and two accessory subunits Med12 and Med13 called the Cdk8-dependent kinase module (CKM). The CKM reversibly associates with the multi-subunit transcriptional coactivator complex, the Mediator, to modulate RNA polymerase II-dependent transcription. Apart from its transcriptional regulatory function, recent research has revealed a novel signaling role for cyclin C at the mitochondria. Upon oxidative stress, cyclin C leaves the nucleus and directly activates the guanosine 5'-triphosphatase (GTPase) Drp1, or Dnm1 in yeast, to induce mitochondrial fragmentation. Importantly, cyclin C-induced mitochondrial fission was found to increase sensitivity of both mammalian and yeast cells to apoptosis. Here, we review and discuss the biology of cyclin C, focusing mainly on its transcriptional and non-transcriptional roles in tumor promotion or suppression.

Keywords: Cdk8-dependent kinase module; Mediator; cancer; cyclin family; stress signaling; transcriptional cyclins; tumor suppressor; yeast.

Figure 1

Domain composition of cyclin primary sequences and their relative similarity to cyclin C. Primary sequences of representative cyclins were annotated for domain-specific features including f-box (pink), cyclin N2 domain (blue), cyclin box fold N (gray) and C (brown). Relative sequence homology between cyclin C and other cyclins is shown on the right. Values, representing the number of identical or similar amino acid residues divided by the total protein length, were generated using EMBOSS Needle and expressed as percentages [38].

Figure 2

Phylogenetic analysis of eukaryotic cyclin sequences. Phylogenetic tree was generated using Basic Local Alignment Search Tool (BLAST). Members of the human cyclin family are indicated with an asterisk. All other sequences are cyclin C orthologs. CCL1 is a yeast cyclin H paralog. S. cerevisiae Ume3/Srb11/Ssn8 and S. pombe cyclin C homologs are the most highly conserved cyclins within this multiple sequence alignment. Scale bar indicates number of amino acid substitutions per site.

Figure 3

Sequence homology comparison between human and yeast cyclin C. Shown is the alignment of the primary sequences of human and yeast cyclin C. Grey and brown boxes denote α-helical segments of the N- and C-terminal cyclin box domains, respectively. Shown is the HAD domain (pink), the two amino acid residues Tyr76 (Ala110 in yeast) (magenta) and Leu145 (Glu170 in yeast) (orange) required for Cdk8 binding, and five highly conserved residues at the surface of a cyclin C-specific groove between the two cyclin box domains (human: Ile42, Arg58, Trp177, Asp182, and Tyr184) (yellow) (see Section 5.1.1) [47].

Figure 4

Crystal structure of human cyclin C-Cdk8 heterodimer. The ribbon model of cyclin C (cyan) and Cdk8 (grey) was generated using PyMol, PDB code 3RGF [48]. Highlighted is the conserved N-terminal KERQK sequence (red) within the HAD domain (pink) and two amino acid residues of cyclin C critical for establishing interaction with Cdk8, Tyr76 (Ala110 in yeast) (magenta) and Leu145 (Glu170 in yeast) (orange) [47]. The αB helix (green) and αC helix (blue) of Cdk8, which are responsible for making contact with the N-terminal cyclin box domain, and five highly conserved residues at the surface of a cyclin C-specific groove between the two cyclin folds (Ile42, Arg58, Trp177, Asp182, and Tyr184) (see Section 5.1.1) (yellow) are also indicated.

Figure 5

Day job versus night job function of cyclin C. (Left) Under normal conditions, cyclin C performs its day job as a nuclear transcription factor that regulates RNA Pol II-dependent activity. In unstressed cells, mitochondria maintain a reticular structure [33]. (Right) The night job function of cyclin C is initiated by cellular stress and is associated with the activation of two layers of stress defense. First, under adverse conditions such as oxidative stress, a portion of cyclin C exits the nucleus to induce Drp1-dependent mitochondrial scission. The resulting fragmentation of mitochondrial network facilitates downstream stress responses such as mitophagy or apoptosis. Second, upon dissociation of cyclin C from the promoters of stress response genes, the activity of RNA Pol II is regained, which triggers a host of adaptive mechanisms such as antioxidant defense or unfolded protein response.

Figure 6

Cyclin C-Cdk8/19/3 inhibit the Notch pathway by phosphorylating the Notch intracellular fragment (NICD) and promoting its degradation in leukemic T cells. In canonical Notch signaling, a contiguous cell exposes a Notch ligand which is recognized by a Notch receptor paralog on a receiving cell. Subsequently, a two-step proteolytic cleavage produces NICD which stimulates with a transcriptionally paused RNA Pol II complex resulting in the downstream expression of genes involved in stem cell differentiation and self-renewal.