Biochemical Study Of Archaeal Presenilin Homologue And Identification Of Human Presenilin Binding Protein

Presenilin(PS)is one of the key genes involved in the pathogenesis of Alzheimer's disease.More than 200 familial Alzheimer's disease(FAD)derived mutations are identified in PS.PS is the catalytic subunit of the intramembrane aspartyl protease complex ?-secretase,assembling into the mature ?-secretase together with the other three components Aph-1,Pen-2 and nicastrin(NCT).?-secretase is responsible for the cleavage of amyloid precursor protein(APP)into ?-amyloid(A?)peptides,which are the major component of amyloid plaques derived from patients' brain.The first part of my work focuses on the cleavage activity towards APP C99 by archaeal presenilin homologue(PSH).We fi nd that,similar to ?-secretase,PSH faithfully processes the substrate APP C99 into A?42,A?40,A?38.The molar ratio of the cleavage products A?42 over A?40 by PSH is nearly identical to that by ?-secretase.The proteolytic activity of PSH is specifically suppressed by presenilin-specific inhibitors.Known modulators of ?-secretase also modulate PSH similarly in terms of the A?42/ A?40 ratio.Structural analysis reveals association of a known ?-secretase inhibitor with PSH between its two catalytic aspartate residues.Our work reveals that,PSH represents an excellent surrogate of ?-secretase in terms of biochemical properties,and may facilitate the development and screening of agents that regulate the protease activity and the cleavage preference of ?-secretase.The second part of my work concerns directly the human presenilin 1(PS1),seeking to investigate the potential alternative Alzheimer's disease pathogenesis pathway by identification of PS interacting protein other than ?-secretase components.Mass spectroscopy analysis indicates that Bax-inhibitor 1(BI1),an evolutionarily-conserved membrane protein,associates with PS1 in isolation.We refer to this putative complex as PS1-BI.BI1 specifically interacts with PS1 in isolation,but not PS1 in the context of an assembled ?-secretase.The PS1-BI1 complex exhibits no apparent proteolytic activity as judged by the production of A?40 and A?42 from the substrate APP C99.At an equimolar concentration,BI1 has no impact on the proteolytic activity of ?-secretase;at 150-fold molar excess,BI1 cripples ?-secretase.In cellular context,BI1 co-localizes with PS1 holoprotein,but separates from PS1 as assembled mature ?-secretase complex.These results identify BI1 as a novel PS1-interacting protein,suggesting additional functions of PS1 beyond its involvement of ?-secretase.Our finding may provide a new framework for the understanding of the pathogenesis process of Alzheimer's disease.