Mechanisms and Function of Intramembrane Proteolysis by the gramme-Secretase homologous Signal Peptide Peptidase-like Proteases (SPPL)

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Intramembrane proteolysis was thought to be biochemically impossible, because each protease requires water for its catalytic activity. However, upon investigation of the proteolytic generation of the Alzheimer¿s disease associated Amyloid ß-peptide it became clear that ¿-secretase can indeed cut in the middle of the membrane in a hydrophobic environment under physiological conditions. Whereas ¿-secretase exclusively cuts type 1 trans-membrane domain proteins, signal peptide peptidase (SPP) only accepts type 2 oriented proteins as substrates. Data bank analysis lead to the identification of a family of SPP-like proteins (SPPL2a,b,c and SPPL3), whose function is until now completely unknown. Very recently we were able to demonstrate that SPPLs, like ¿-secretase, belong to the GxGD type-aspartyl proteases, which we originally defined as a new class of intramembrane cleaving proteases. We are now interested to understand the function of the SPPLs and the mechanisms by which they are able to cleave within the membrane. We will identify their substrates and will subsequently investigate substrate processing in vivo and in vitro to determine the cleavage sites, cleavage site requirements and a putative mechanistic analogy to ¿-secretase. Substrates will be verified by rescuing knock down phenotypes in an animal model (zebrafish) upon expression of the recombinant cleavage products. Our findings will contribute to the understanding of the novel scheme of intramembrane proteolysis by GxGD proteases, which appears to be of great importance in pivotal physiological and pathological processes, such as Alzheimer¿s disease, Cholesterol metabolism, Notch-signaling, removal of signal peptides, immune surveillance, and processing of the Hepatitis C viral core protein.