Ubiquitin, a small protein consisting of 76 amino acids, has been found in all eukaryotic
cells studied. It is one of the most conserved proteins known; the amino acid sequence is
identical from insects to humans, and there are only 3 substitutions within the plant and
yeast sequences. Two classes of ubiquitin genes are recognized. Class I is a polyubiquitin
gene encoding a polyprotein of tandemly repeated ubiquitins . The class II
genes are fusion products between a single ubiquitin gene and 1 of 2 other possible
sequences, either 52 or 76 to 80 predominantly basic amino acids. Ubiquitin is required for
ATP-dependent, nonlysosomal intracellular protein degradation, which eliminates most
intracellular defective problems as well as normal proteins with a rapid turnover.
Degradation involves covalent binding of ubiquitin to the protein to be degraded, through
isopeptide bonds from the C-terminal glycine residue to the epsilon-amino groups of lysyl
side chains.
NCBI Summary:
Ubiquitin-like proteins (UBLs) are thought to be reversible modulators of protein function rather than protein degraders like ubiquitin (MIM 191339).[supplied by OMIM]
General function
Cell cycle regulation
Comment
Cellular localization
Cytoplasmic
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Ovarian function
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Expression regulated by
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Ovarian localization
Oocyte
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Sutovsky et al. (1999) demonstrated that sperm mitochondria are selectively marked for
destruction by a ubiquitin tag. In fertilized eggs from rhesus monkeys and cows the
ubiquitination was evident at first mitosis.
Participation of the ubiquitin-proteasome pathway in rat oocyte activation Tan X, et al .
The role of the ubiquitin-proteasome pathway (UPP) in mitosis is well known. However, its role in meiotic division is still poorly documented, especially in the activation of mammalian oocytes. In this study, the role of proteasome in the spontaneous and parthenogenetic activation of rat oocytes was investigated. We found that ALLN, an inhibitor of proteasome, when applied to metaphase II oocytes, inhibited spontaneous activation, blocked extrusion of the second polar body (PB) and caused the withdrawal of the partially extruded second PB. ALLN also inhibited the parthenogenetic activation induced by cycloheximide, but had no effect on the formation of pronuclei in activated eggs. In metaphase and anaphase, ubiquitin and proteasome localized to the meiotic spindle, concentrating on both sides of the oocyte-second PB boundary during PB extrusion. This pattern of cellular distribution suggests that UPP may have a role in regulating nuclear division and cytokinesis. Ubiquitin was seen to form a ring around the pronucleus, whereas proteasome was evenly distributed in the pronuclear region. Taken together, our results indicate that (1) UPP is required for the transitions of oocytes from metaphase II to anaphase II and from anaphase II to the end of meiosis; and (2) the UPP plays a role in cytokinesis of the second meiotic division.