Two secretases are involved in the generation of amyloid beta-peptide, the 
principal component of amyloid plaques in the brains of Alzheimer's disease 
patients. While beta-secretase is a classical aspartyl protease, gamma-secretase 
activity is associated with a high molecular weight complex. One of the complex 
components, which is critically required for gamma-secretase activity is 
nicastrin (NCT). Here we investigate the assembly of NCT into the 
gamma-secretase complex. NCT mutants either lacking the entire cytoplasmic tail, 
the cytoplasmic tail, and the transmembrane domain (TMD), or containing a set of 
heterologous TMDs were expressed in cells with strongly reduced levels of 
endogenous NCT. Maturation of exogenous NCT, gamma-secretase complex formation 
and proteolytic function was then investigated. This revealed that the 
cytoplasmic tail of NCT is dispensable for gamma-secretase complex assembly and 
function. In contrast, the authentic TMD of NCT is critically required for the 
interaction with gamma-secretase complex components and for formation of an 
active gamma-secretase complex. Neither soluble NCT lacking any membrane anchor 
nor NCT containing a heterologous TMD were inserted into the gamma-secretase 
complex. We identified the N-terminal region of the NCT TMD as a functionally 
important entity of NCT. These data thus demonstrate that NCT interacts with 
other gamma-secretase complex components via its TMD.