Solution structure and biochemistry of an intramembrane aspartyl protease (IAP)
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Intramembrane proteases (IPs) play important roles in numerous biochemical processes in all kingdoms of life, including cell differentiation, development and metabolism. IPs are attractive targets for therapeutics development and have been implicated in the pathogenesis of human diseases, such as Alzheimer’s disease, Hepatitis C, malaria and a variety of cancers. Despite their broad medical and biological relevance, structural and mechanistic details of IPs have remained largely obscure. A continuous FRET peptide assay, which is fast, robust, and high throughput, and can be applied to screen multiple conditions simultaneously, is developed. Catalytic parameters and cleavage sites of a fortuitous substrate, angiotensinogen, and Alzheimer disease linked C100 substrate, in detergent and bicelle conditions highlighted the importance of substrate-membrane, substrate-enzyme and enzyme-membrane interactions. Through structure-function analysis of a model IAP from the archaeon Methanoculleus marisnigri JR1 (mIAP), the catalytic motifs and substrate gating motifs were found to influence the catalytic efficiency but not the processivity and specificity of cleavages. Both positional and chemical variables were found to control the substrate specificity of mIAP. Small-angle neutron scattering (SANS) study showed that mIAP is a compact monomer in solution. Future direction of study on expanding substrate repertoire, evaluating effects of Alzheimer disease mutations, and structural characterization of substrate-enzyme-membrane complexes are discussed.