Cellular Functions of -Secretase-Related Proteins

Amyloidpeptide (A ) is generated by -secretase, a membrane protein complex with an unusual aspartyl protease activity consisting of the four components presenilin, nicastrin, APH-1 and PEN-2. Presenilin is considered the catalytic subunit of this complex since it represents the prototype of the new family of intramembrane-cleaving GxGD-type aspartyl proteases. Recently, five novel members of this family and a nicastrin-like protein were identified. Whereas one of the GxGD-type proteins was shown to be identical with signal peptide peptidase (SPP), the function of the others, now called SPP-like proteins (SPPLs), is not known. We therefore analyzed SPPL2b and SPPL3 and demonstrated that they localize to different subcellular compartments suggesting nonredundant functions. This was supported by different phenotypes obtained in knockdown studies in zebrafish embryos. In addition, these phenotypes could be phenocopied by ectopic expression of putative active site mutants, providing strong evidence for a proteolytic function of SPPL2b and SPPL3. We also identified and characterized the nicastrin-like protein nicalin which, together with the 130kDa protein NOMO (Nodal modulator), forms a membrane D i s e a s e s Christof Haffner, Laboratory for Alzheimer’s and Parkinson’s Disease Research Department of Biochemistry, Adolf-Butenandt-Institute Ludwig-Maximilians-University, DE–80336 Munich (Germany) Tel. +49 89 218 075 484, Fax +49 89 218 075 415 E-Mail [email protected] © 2006 S. Karger AG, Basel 1660–2854/06/0035–0284$23.50/0 Accessible online at: www.karger.com/ndd Cellular Functions of -Secretase-Related Proteins Neurodegenerative Dis 2006;3:284–289 285 be the pathological agent that initiates AD. The physiological role of the APP processing is not known, but secretase-mediated cleavage of other substrates, most importantly the Notch receptor, is required during development and maintenance of multicellular organisms [3] . Four -secretase complex components have been identified: presenilin, nicastrin, APH-1 and PEN-2 [4] . Their simultaneous expression in yeast, an organism that lacks any endogenous -secretase activity, results in the reconstitution of -secretase complex formation and activity, demonstrating that these four membrane proteins are the core components of the complex [5] . -Secretase activity depends on the presence of two conserved aspartate residues in either presenilin 1 or presenilin 2, two polytopic membrane proteins with partially redundant function [4] . Presenilins constitutively undergo endoproteolysis, leading to the generation of Nand C-terminal fragments which remain bound to each other and apparently constitute the catalytic site of the -secretase. The active site aspartate residues of the presenilins reside within their TMDs 6 and 7, in agreement with an intramembrane cleavage mechanism. The TMD 7 aspartate is located within the highly conserved sequence motif GxGD [6] , which is also found in other intramembrane cleaving proteases, the bacterial type 4 prepilin peptidases and signal peptide peptidase (SPP) (see below). These and other data strongly suggest that the presenilins are the proteolytically active components of the -secretase complex. In contrast, the role of the other three subunits is less well understood. The type I glycoprotein nicastrin (see below) and the polytopic membrane protein APH-1 might form a precomplex to which first presenilin holoprotein is added [7] . The subsequent addition of PEN-2 facilitates presenilin endoproteolysis leading to the generation of the active complex. For a detailed description of -secretase assembly and function see the paper by Kaether et al. [this issue, pp. 275–283].