The protein was originally isolated as part of a trimeric protein complex that could reconstitute procaspase processing and apoptotic events in cell-free assays (hence, the original names given to these proteins were apoptotic protease-activating factors or Apaf-1, Apaf-2 and Apaf-3)(Zou et al., 1997 ). The other two proteins of this complex were identified as cytochrome-c (Apaf-2) and procaspase-9 (Apaf-3). In the presence of cytochrome c and dATP (or ATP), Apaf-1 undergoes a conformational change allowing for heterodimeric interaction with, and oligermization, of procaspase-9 molecules. Activation of procaspase-9 by an induced proximity model is believed to then activate the caspase cascade, leading to the execution phase of apoptosis (Li et al., 1997 and
Srinivasula et al., 1998 ).
NCBI Summary:
This gene encodes a cytoplasmic protein that initiates apoptosis. This protein contains several copies of the WD-40 domain, a caspase recruitment domain (CARD), and an ATPase domain (NB-ARC). Upon binding cytochrome c and dATP, this protein forms an oligomeric apoptosome. The apoptosome binds and cleaves caspase 9 preproprotein, releasing its mature, activated form. Activated caspase 9 stimulates the subsequent caspase cascade that commits the cell to apoptosis. Alternative splicing results in several transcript variants encoding different isoforms.
General function
Cell death/survival, Apoptosis
Comment
Apaf-1, the first identified mammalian counterpart to the C. elegans gene product CED-4, is believed to be a critical molecular bridge or adapter protein that links Bcl-2 family member function to caspase activation (see above). A second C. elegans CED-4 homolog in mammals, termed FLASH, has recently been reported (Imai et al., 1999 ).
Cellular localization
Cytoplasmic, Mitochondrial
Comment
Ovarian function
Follicle atresia, Oogenesis, Oocyte maturation
Comment
A role for Apaf-1 in granulosa cell apoptosis during atresia of maturing antral follicles in the mouse ovary has been proposed from expression analysis (mRNA and protein), localizing accumulation of Apaf-1 protein in granulosa cells of follicles on the verge of atresia. In addition, this same study showed that mitochondrial cytochrome c release, the biochemical step that "activates" Apaf-1, occurs in mouse granulosa cells during apoptosis induction in vitro (Robles et al., 1999 ).
Genes whose expression is detected by cDNA array hybridization: GDP/GTP exchangers, GTPase stimulators and inhibitors, apoptosis Rozenn Dalbis-Tran and Pascal Mermilloda
Expression regulated by
FSH, LH
Comment
In vivo gonadotropin priming of prepubertal female mice has been reported to suppress Apaf-1 expression in granulosa cells, coincident with an inhibition of granulosa cell apoptosis and antral follicle atresia (Robles et al., 1999 ).
Matsui T, et al reported the expression and activity of Apaf1 and caspase-9 in granulosa cells during follicular atresia in pig ovaries.
This study examined whether the mitochondrial pathway is associated with granulosa cell apoptosis during atresia in pig ovaries. Both mRNAs of caspase-9 and apoptotic protease-activating factor 1 (Apaf1), which are major signal transducing components in the mitochondrial pathway, were detected in granulosa cells in healthy, early atretic and progressed atretic follicles by RT-PCR. No changes in the expression of Apaf1 mRNA were seen during follicular atresia, but the expression of caspase-9 mRNA increased during atresia. Apaf1 protein was steadily detected in granulosa cells prepared from healthy, early atretic and progressed atretic follicles by western blot analysis, but high expression of the precursor of caspase-9 (procaspase-9) was detected only in granulosa cells of healthy follicles. Decreased procaspase-9 protein was demonstrated during follicular atresia. Proteolytic activity of caspase-9 increased during atresia, in agreement with the diminution of procaspase-9 protein. Intensive expression of caspase-9 mRNA was demonstrated in the granulosa cells of early atretic and progressed atretic follicles but not in those of healthy follicles. These results indicate that the mitochondrial signalling pathway, which is mediated by Apaf1 and caspase-9, plays a crucial role in determining the fate of granulosa cells during atresia in pig ovaries.
Phenotypes
Mutations
1 mutations
Species: mouse
Mutation name: apaf1 gene disruption
type: null mutation fertility: unknown Comment: Parallel original reports of targeted inactivation of the apaf1 gene in mice revealed that homozygous null mice die prior to birth due to severe defects in central nervous system development (Cecconi et al., 1998 and Yoshida et al., 1998 ).