Intramembrane proteases (IMPs), also known as intramembrane-cleaving proteases (I-CLiPs), are enzymes that have the property of cleaving transmembrane domains of integral membrane proteins.[1][2][3] All known intramembrane proteases are themselves integral membrane proteins with multiple transmembrane domains, and they have their active sites buried within the lipid bilayer of cellular membranes.[4] Intramembrane proteases are responsible for proteolytic cleavage in the cell signaling process known as regulated intramembrane proteolysis (RIP).[1][5]
Although only recently discovered, intramembrane proteases are of significant research interest because of their major biological functions and their relevance to human disease.[5]
Classification
There are four groups of intramembrane proteases, distinguished by their catalytic mechanism:[5]
Three of the four groups of intramembrane proteases cleave their substrates within transmembrane domains and the scissile bond is located inside the membrane. The remaining group, Rce1 glutamyl proteases, cleaves the C-terminus of CAAX proteins.[17] The kinetics of intramembrane proteases are generally slower than soluble proteases.[18][19]Substrate specificity is not well understood and varies significantly between enzymes, with the gamma-secretase complex in particular known for its substrate promiscuity.[18][20] Both rhomboid protease and gamma-secretase have been reported to have an unusual substrate recognition mechanism by distinguishing substrates from non-substrates only after forming a protein complex, giving rise to their slow enzyme kinetics.[19]
Although soluble proteases are among the earliest and best characterized enzymes, intramembrane proteases were discovered relatively recently.[21][18] Intramembrane proteolysis was proposed in the 1990s by researchers studying Alzheimer's disease, such as Dennis Selkoe, as a possible mechanism for the processing of amyloid precursor protein.[22] The possibility of hydrolysis occurring within the hydrophobic membrane was initially controversial.[21][18] The first intramembrane protease to be experimentally identified was site-2 protease in 1997.[9]
^Urban, S; Freeman, M (October 2002). "Intramembrane proteolysis controls diverse signalling pathways throughout evolution". Current Opinion in Genetics & Development. 12 (5): 512–8. doi:10.1016/s0959-437x(02)00334-9. PMID12200155.
^Wolfe, MS; Kopan, R (20 August 2004). "Intramembrane proteolysis: theme and variations". Science. 305 (5687): 1119–23. doi:10.1126/science.1096187. PMID15326347.
^Erez, E; Fass, D; Bibi, E (21 May 2009). "How intramembrane proteases bury hydrolytic reactions in the membrane". Nature. 459 (7245): 371–8. doi:10.1038/nature08146. PMID19458713.
^ abcdSun, Linfeng; Li, Xiaochun; Shi, Yigong (April 2016). "Structural biology of intramembrane proteases: mechanistic insights from rhomboid and S2P to γ-secretase". Current Opinion in Structural Biology. 37: 97–107. doi:10.1016/j.sbi.2015.12.008.
^ abcdBeard, Hester A.; Barniol-Xicota, Marta; Yang, Jian; Verhelst, Steven H. L. (15 November 2019). "Discovery of Cellular Roles of Intramembrane Proteases". ACS Chemical Biology. 14 (11): 2372–2388. doi:10.1021/acschembio.9b00404.
^ abPaschkowsky, Sandra; Hsiao, Jacqueline Melissa; Young, Jason C.; Munter, Lisa Marie (June 2019). "The discovery of proteases and intramembrane proteolysis". Biochemistry and Cell Biology. 97 (3): 265–269. doi:10.1139/bcb-2018-0186.