Signal peptide peptidase (SPP) assembles with substrates and misfolded membrane proteins into distinct oligomeric complexes

Overview & Synopsis

Signal peptide peptidase (SPP) is an aspartyl intramembrane cleaving protease, which processes a subset of signal peptides, and is linked to the quality control of endoplasmic reticulum (ER) membrane proteins. We analysed SPP interactions with signal peptides and other membrane proteins by co-immunoprecipitation assays. We found that SPP specifically and tightly interacts with a large range of newly synthesized membrane proteins, including signal peptides, preproteins and misfolded membrane proteins, but not with all co-expressed type II membrane proteins. Signal peptides are trapped by the catalytically inactive SPP mutant SPPD/A. Preproteins and misfolded membrane proteins interact with both, SPP and the SPPD/A mutant, and are not substrates for SPP-mediated intramembrane proteolysis. Proteins interacting with SPP are found in distinct complexes of different sizes. A signal peptide is mainly trapped in a 200 kDa SPP complex, while a preprotein is predominantly found in a 600 kDa SPP complex. A misfolded membrane protein is detected in 200 kDa, 400 kDa and 600 kDa SPP complexes. We conclude that SPP does not only process signal peptides but also collects preproteins and misfolded membrane proteins that are destined for disposal.