The final step in Notch signaling activation is mediated by gamma-secretase cleavage of the Notch receptor. Gamma secretase consists of four evolutionarily conserved integral membrane subunits that are considered obligate components for enzyme activity in vivo: presenilin (SEL-12/HOP-1), APH-1, PEN-2, and APH-2/Nicastrin. The presenilin subunit contains the catalytic site, and can execute substrate cleavage in vitro, leaving the essential role of the other three subunits more mysterious. Current models suggest that the three subunits are necessary for enzyme maturation and stability and/or substrate selection. Here we present evidence that challenges the quartet model of gamma secretase, and propose that in certain contexts, such as the C. elegans germline, the APH-2 subunit is dispensable. We used CRISPR-Cas9 to generate the
aph-2(
ik7) deletion, and show that homozygous
aph-2(
ik7) animals display the classic Notch phenotypes associated with
glp-1 and
lin-12 lof mutants, except that they successfully maintain a
glp-1-dependent germline proliferative zone throughout adulthood. In contrast, animals with mutations in any of the other three subunits of gamma secretase are unable to sustain Notch signaling in the adult germline. The requirement for
aph-2 function in establishment of the larval germline was tested by using RNAi to deplete both maternal and zygotic gene products. In contrast to
aph-1(RNAi) treatment, which caused the expected Notch phenotype of sterility,
aph-2(RNAi) treatment had no effect on germline establishment. These results contradict the accepted view that APH-2 is necessary for gamma secretase-mediated activation of Notch. To further test the need for APH-2, we reconstituted the C. elegans gamma secretase complex in yeast, and found that a three-part complex lacking APH-2 could successfully, albeit less efficiently, cleave a Notch-like substrate. We propose a revised model of gamma secretase where the APH-2 subunit may only be necessary in certain cellular contexts. Unique features of the germline, such as cellular architecture, persistence of ligand presentation, abundance of receptor, or a lower threshold for activated Notch, may place different demands on gamma secretase activity such that APH-2 is dispensable for Notch signaling in the germline, but required in other tissues.