GSAP, or gamma-secretase-activating protein, also known as PION, regulates gamma-secretase activity. The holo-protein is a large, approx 850 residue protein that is rapidly cleaved to an active 16 kDa C-terminal fragment that is the stable, predominant form. GSAP is expressed in inclusion bodies and is important in brain function. It dramatically and selectively increases neurotoxic beta-Amyloid production in the brain through a mechanism involving its interactions with both gamma-secretase and its substrate, the amyloid precursor protein C-terminal fragment (APP-CTF). Accumulation of neurotoxic beta-Amyloid is a major hallmark of Alzheimer's disease. Formation of beta-Amyloid is catalysed by gamma-secretase, a protease with numerous substrates that catalyses the intra-membrane cleavage of integral membrane proteins such as Notch receptors and APP (beta-amyloid precursor protein) [1]. The secondary structure of GSAP is largely alpha-helical, lacking well-defined tertiary structure. GSAP represents a type of gamma-secretase regulator that directs enzyme specificity by interacting with a specific substrate [2].

1: Gamma-secretase activating protein is a therapeutic target for Alzheimer's disease. He G, Luo W, Li P, Remmers C, Netzer WJ, Hendrick J, Bettayeb K, Flajolet M, Gorelick F, Wennogle LP, Greengard P; Nature. 2010;467:95-98. PMID:20811458

2: Purification and Characterization of the Human gamma-Secretase Activating Protein. Deatherage CL, Hadziselimovic A, Sanders CR; Biochemistry. 2012; [Epub ahead of print] PMID:22681044