The Alzheimer disease-associated presenilin (PS) proteins apparently provide the active site of gamma-secretase, an unusual intramembrane-cleaving aspartyl protease. PSs principally occur as high molecular weight protein complexes that contain nicastrin (Nct) and additional so far unidentified components. Recently, PEN-2 has been implicated in gamma-secretase function. Here we identify PEN-2 as a critical component of PS1/gamma-secretase and PS2/gamma-secretase complexes. Strikingly, in the absence of PS1 and PS1/PS2, PEN-2 levels are strongly reduced. Similarly, PEN-2 levels are reduced upon RNA interference-mediated down-regulation of Nct. On the other side, down-regulation of PEN-2 by RNA interference is associated with reduced PS levels, impaired Nct maturation, and deficient gamma-secretase complex formation. We conclude that PEN-2 is an integral gamma-secretase complex component and that gamma-secretase complex components are expressed in a coordinated manner.