Topology surveillance of the lanosterol demethylase CYP51A1 by Signal Peptide Peptidase.

Cleavage of transmembrane segments on target proteins by the aspartyl intramembrane protease signal peptide peptidase (SPP) has been linked to immunity, viral infection and protein quality control. How SPP recognizes its various substrates and specifies their fate remains elusive. Here we identified the lanosterol demethylase CYP51A1 as an SPP substrate and show that SPP-catalyzed cleavage triggers CYP51A1 clearance by ER-associated degradation (ERAD). We observe that SPP targets only a fraction of CYP51A1 molecules and identified an amphipathic helix in the N-terminus as a key determinant for SPP recognition. SPP recognition is remarkably specific to CYP51A1 molecules with the amphipathic helix aberrantly inserted in the membrane with a type II orientation. Thus, our data are consistent with a role for SPP in topology surveillance, triggering the clearance of certain, potentially non-functional conformers.