Effects of membrane lipids on the activity and processivity of purified ?-secretase.

ABSTRACT: The 19-transmembrane multisubunit ?-secretase complex generates the amyloid ?-peptide (A?) of Alzheimer's disease (AD) by intramembrane proteolysis of the ?-amyloid precursor protein (APP). Despite substantial advances in elucidating how this protein complex functions, the effect of the local membrane lipid microenvironment on ?-secretase cleavage of substrates is still poorly understood. Using detergent-free proteoliposomes to reconstitute purified human ?-secretase, we examined the effects of fatty acyl (FA) chain length, saturation and double-bond isomerization, and membrane lipid polar headgroups on ?-secretase function. We analyzed ?-secretase activity and processivity [i.e., sequential cleavages in the APP transmembrane domain that convert longer A? species (e.g., A?(46)) into shorter ones (e.g., A?(40))] by quantifying the APP intracellular domain (AICD) and various A? peptides, including via a bicine/urea gel system that detects multiple A? lengths. These assays revealed several trends. (1) Switching from a cis to a trans isomer of a monounsaturated FA chain in phosphatidylcholine (PC) increased ?-activity, did not affect A?(42):A?(40) ratios, but decreased the ratio of long (?42) versus short (?41) A? peptides. (2) Increasing the FA carbon chain length (14, 16, 18, and 20) increased ?-activity, reduced longer A? species, and reduced the A?(42):A?(40) ratio. (3) Shifting the position of the double bond in 18:1(?9-cis) PC to the ?6 position substantially reduced activity. (4) Gangliosides increased ?-activity but decreased processivity, thus elevating the A?(42):A?(40) ratio. (5) Phosphatidylserine decreased ?-activity but increased processivity. (6) Phosphatidylinositol strongly inhibited ?-activity. Overall, our results show that subtle changes in membrane lipid composition can greatly influence ?-secretase activity and processivity, suggesting that relatively small changes in lipid membrane composition may affect the risk of AD at least as much as presenilin or APP mutations do.

SUBMITTER: Holmes O 

PROVIDER: S-EPMC3347702 | biostudies-literature | 2012 May

REPOSITORIES: biostudies-literature