SUBCELLULAR-DISTRIBUTION AND TURNOVER OF PRESENILINS IN TRANSFECTED CELLS

The mechanisms by which mutations in presenilin-1 (PS1) and presenilin -2 (PS2) result in the Alzheimer's disease phenotype are unclear. Full -length PS1 and PS2 are each processed into stable proteolytic fragmen ts after their biosynthesis in transfected cells. PS1 and PS2 have bee n localized by immunocytochemistry to the endoplasmic reticulum (ER) a nd Golgi compartments, but previous studies could not differentiate be tween the full-length presenilin proteins and their fragments. We carr ied out subcellular fractionation of cells stably transfected with PS1 or PS2 to determine the localization of full-length presenilins and t heir fragments. Full-length PS1 and PS2 were principally distributed i n ER fractions, whereas the N- and C-terminal fragments were localized predominantly to the Gels fractions. In cells expressing the PS1 muta nt lacking exon 9 (Delta E9), we observed only full-length molecules t hat were present in the ER and Gels fractions. The turnover rate was c onsiderably slower for the Delta E9 holoprotein, apparently due to dec reased degradation within the ER. Our results suggest that that full-l ength presenilin proteins are primarily ER resident molecules and unde rgo endoproteolysis within the ER. The fragments are subsequently tran sported to the Golgi compartment, where their turnover rate is much sl ower than that of the full-length presenilin in the ER.