Mutations in presenilin (PS) 1 or PS2 genes account for the majority of early-onset familial Alzheimer's disease, and these mutations have been shown to increase production of species of amyloid beta peptide (Abeta) ending at residue 42, i.e. the most amyloidogenic form of Abeta. To gain insight into the molecular mechanisms whereby mutant PS induces overproduction of Abeta42, we constructed cDNAs encoding mutant and/or truncated forms of PS2 and examined the secretion of Abeta42 from COS or neuro2a cells transfected with these genes. Cells expressing full-length PS2 harboring both N141I and M239V mutations in the same polypeptide induced overproduction of Abeta42, although the levels of Abeta42 were comparable with those in cells engineered to express PS2 with one or the other of these PS2 mutations. In contrast, cells engineered to express partially truncated PS2 (eliminating the COOH-terminal third of PS2 while retaining the endoproteolytic NH2-terminal fragment) and harboring a N141I mutation, as well as cells expressing COOH-terminal fragments of PS2, did not overproduce Abeta42, and the levels of Abeta42 were comparable with those in cells that expressed full-length, wild-type PS2 or fragments thereof. These data indicate that: (i) the Abeta42-promoting effects of mutant PS2 proteins reach the maximum level with a given single amino acid substitution (i.e. N141I or M239V); and (ii) the expression of full-length mutant PS2 is required for the overproduction of Abeta42. Hence, cooperative interactions of NH2- and COOH-terminal fragments generated from full-length mutant PS2 may be important for the overproduction of Abeta42 that may underlie familial Alzheimer's disease.