Regulation of APC/C activity in oocytes by a Bub1-dependent spindle assembly checkpoint.
BACKGROUND
Missegregation of chromosomes during meiosis in human females causes aneuploidy, including trisomy 21, and is thought also to be the major cause of age-related infertility. Most errors are thought to occur at the first meiotic division. The high frequency of errors raises questions as to whether the surveillance mechanism known as the spindle assembly checkpoint (SAC) that controls the anaphase-promoting complex or cyclosome (APC/C) operates effectively in oocytes. Experimental approaches hitherto used to inactivate the SAC in oocytes suffer from a number of drawbacks.


RESULTS
Bub1 protein was depleted specifically in oocytes with a Zp3-Cre transgene to delete exons 7 and 8 from a floxed BUB1(F) allele. Loss of Bub1 greatly accelerates resolution of chiasmata and extrusion of polar bodies. It also causes defective biorientation of bivalents, massive chromosome missegregation at meiosis I, and precocious loss of cohesion between sister centromeres. By using a quantitative assay for APC/C-mediated securin destruction, we show that the APC/C is activated in an exponential fashion, with activity peaking 12-13 hr after GVBD, and that this process is advanced by 5 hr in oocytes lacking Bub1. Importantly, premature chiasmata resolution does not occur in Bub1-deficient oocytes also lacking either the APC/C's Apc2 subunit or separase. Finally, we show that Bub1's kinase domain is not required to delay APC/C activation.


CONCLUSIONS
We conclude that far from being absent or ineffective, the SAC largely determines the timing of APC/C and hence separase activation in oocytes, delaying it for about 5 hr.
McGuinness
Barry E
BE
Department of Biochemistry, University of Oxford, South Parks Road, Oxford, OX1 3QU, United Kingdom.
Anger
Martin
M
Kouznetsova
Anna
A
Gil-Bernabé
Ana M
AM
Helmhart
Wolfgang
W
Kudo
Nobuaki R
NR
Wuensche
Annelie
A
Taylor
Stephen
S
Hoog
Christer
C
Novak
Bela
B
Nasmyth
Kim
K
D064173:Anaphase-Promoting Complex-Cyclosome; D000818:Animals; D064190:Apc2 Subunit, Anaphase-Promoting Complex-Cyclosome; D018797:Cell Cycle Proteins / Q000378:metabolism; D020090:Chromosome Segregation; D010450:Endopeptidases / Q000378:metabolism; D004789:Enzyme Activation; D005260:Female; D006801:Humans; D008297:Male; D008540:Meiosis / Q000502:physiology; D051379:Mice; D008822:Mice, Transgenic; D009865:Oocytes / Q000166:cytology / Q000502:physiology*; D011247:Pregnancy; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D064247:Separase; D008941:Spindle Apparatus / Q000378:metabolism*; D043743:Ubiquitin-Protein Ligase Complexes / Q000235:genetics / Q000378:metabolism*
D018797:Cell Cycle Proteins; D043743:Ubiquitin-Protein Ligase Complexes; C577341:Anapc2 protein, mouse; D064173:Anaphase-Promoting Complex-Cyclosome; D064190:Apc2 Subunit, Anaphase-Promoting Complex-Cyclosome; C584277:BUB1 protein, human; C584281:Bub1 protein, mouse; C090464:Bub1 spindle checkpoint protein; D017346:Protein Serine-Threonine Kinases; D010450:Endopeptidases; C577987:ESPL1 protein, human; D064247:Separase