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General Comment |
NCBI Summary:
The 26S proteasome is a multicatalytic proteinase complex with a highly ordered structure composed of 2 complexes, a 20S core and a 19S regulator. The 20S core is composed of 4 rings of 28 non-identical subunits; 2 rings are composed of 7 alpha subunits and 2 rings are composed of 7 beta subunits. The 19S regulator is composed of a base, which contains 6 ATPase subunits and 2 non-ATPase subunits, and a lid, which contains up to 10 non-ATPase subunits. Proteasomes are distributed throughout eukaryotic cells at a high concentration and cleave peptides in an ATP/ubiquitin-dependent process in a non-lysosomal pathway. An essential function of a modified proteasome, the immunoproteasome, is the processing of class I MHC peptides. This gene encodes a non-ATPase subunit of the 19S regulator. [provided by RefSeq]
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Comment |
Spatial regulation of APCCdh1-induced cyclin B1 degradation maintains G2 arrest in mouse oocytes. Holt JE et al. Within the mammalian ovary, oocytes remain arrested at G2 for several years. Then a peri-ovulatory hormonal cue triggers meiotic resumption by releasing an inhibitory phosphorylation on the kinase Cdk1. G2 arrest, however, also requires control in the concentrations of the Cdk1-binding partner cyclin B1, a process achieved by anaphase-promoting complex (APC(Cdh1) activity, which ubiquitylates and so targets cyclin B1 for degradation. Thus, APC(Cdh1) activity prevents precocious meiotic entry by promoting cyclin B1 degradation. However, it remains unresolved how cyclin B1 levels are suppressed sufficiently to maintain arrest but not so low that they make oocytes hormonally insensitive. Here, we examined spatial control of this process by determining the intracellular location of the proteins involved and using nuclear-targeted cyclin B1. We found that raising nuclear cyclin B1 concentrations, an event normally observed in the minutes before nuclear envelope breakdown, was a very effective method of inducing the G2/M transition. Oocytes expressed only the alpha-isoform of Cdh1, which was predominantly nuclear, as were Cdc27 and Psmd11, core components of the APC and the 26S proteasome, respectively. Furthermore, APC(Cdh1) activity appeared higher in the nucleus, as nuclear-targeted cyclin B1 was degraded at twice the rate of wild-type cyclin B1. We propose a simple spatial model of G2 arrest in which nuclear APC(Cdh1)-proteasomal activity guards against any cyclin B1 accumulation mediated by nuclear import.
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