SCF (Skp1-Cullin 1-F-box) ligases, which are E3 ubiquitin multi-protein enzymes, catalyse protein ubiquitination and thus allow protein degradation mediated by the 26S proteasome. They play a crucial role in the degradation of cell cycle regulators, regulation of the DNA repair and centrosome cycle and play an important role in several diseases. SCF ligases seem to be needed during all phases of development, from oocyte formation through fertilization, activation of the embryonic genome to embryo implantation.
1. Introduction
Mammalian oogenesis and embryogenesis are extremely important processes. Primordial germ cells are converted into oogonia and subsequently to oocytes, and the diploid cell is transformed into a haploid oocyte ready for fertilization. The oocyte grows and accumulates organelles, mRNA and proteins, which are used during the first embryonic stages
[1]. During oogenesis, the oocyte is arrested at the diplotene stage of prophase meiosis I in a structure called a germinal vesicle (GV). Just before ovulation, after the increase in gonadotropin hormones and activation of cyclin dependent kinase (CDK1, a catalytic subunit of the M phase-promoting factor MPF), meiosis is reactivated and the nuclear membrane breaks in a process called germinal vesicle breakdown (GVBD). Subsequently, cytokinesis occurs and the first polar body is extruded. The oocyte enters the second meiotic division and arrest in metaphase II (MII), where the oocyte remains until its fertilization
[2]. Only matured oocytes are able to undergo fertilization. Contact with sperm causes intracellular calcium ion oscillation, leading to oocyte activation and meiosis initiation
[3]. During fertilization, the haploid oocyte and haploid sperm fuse together to create a zygote, and the first mitotic division occurs within several hours
[4]. Early preimplantation development is a highly complicated and strictly regulated process. Preimplantation development starts with several rapid cell cycles with short or completely lacking G phases
[5]. These early stages of development are controlled by maternal mRNAs and proteins accumulated during oogenesis. Subsequently, the control of development passes from maternal to embryonic in a process called the maternal-to-zygotic transition (MZT), leading to embryonic genome activation (EGA)
[6]. A minor wave of EGA shortly after fertilization is followed by a more robust major wave later in development. The first transcripts were found only 7 h after pronuclei formation in mice
[7]. The latest studies suggest that embryonic genome activation is a gradual process that appear with smaller waves of transcription. For example, splicing factor arginine/serine-rich 3 (SRFS3) was found to be already expressed at the 4-cell stage, thus during the minor genome activation in cattle
[8]. Maternal reserves are gradually replaced by their embryonic forms and removed from the embryo. Maternal mRNAs are presumably degraded by miRNAs and probably also by other classes of small non-coding RNAs in early embryos
[9]. One of the potential pathways of maternal protein degradation is via the ubiquitin-proteasome system (UPS). UPS plays a crucial role in many steps of gametogenesis and embryogenesis.
UPS mediates the proteolysis of a variety of proteins that are important for many basic cellular processes, which include the regulation of cell cycle and development, response to stress, DNA repair, modulation of surface receptors and channels, regulation of transcription and many others. This wide range of processes is due to the large number of individual enzymes that belong to the UPS, which affect a huge number of target substrates. The degradation of proteins by the UPS is based on labelling the targeted protein with ubiquitin. This process is called ubiquitination and involves three enzymatic complexes: E1 ubiquitin-activating enzyme, E2 ubiquitin-conjugating enzyme and E3 ubiquitin ligases (
Figure 1). The E3 enzyme mediates the final interaction between ubiquitin and the substrate, and hence is responsible for substrate specificity. Proteins are polyubiquitinated (labelled with more than just one molecule of ubiquitin) and subsequently directed to degradation by the 26S proteasome
[10]. Based on the presence of characteristic domains and mechanisms of ubiquitin transfer from E2 enzyme to the specific substrate proteins, E3 ligases are divided to the three main groups: RING (really interesting new gene) E3s, HECT (homologous to the E6AP carboxyl terminus) E3s and RBR (RING-betweenRING-RING) E3s
[11]. One of the most abundant and common RING E3 enzymes is SCF (Skp1-Cullin 1-F-box) ligase, composed of three invariant members: cullin1, SKP1, RBX1 and one of the F-box proteins, which determines the substrate specificity (
Figure 2).
This entry is adapted from the peer-reviewed paper 10.3390/cells11020234