Functional Relevance of a Novel SlyX Motif in Non-conventional Secretion of Insulin-degrading Enzyme

Insulin-degrading enzyme (IDE) is a Zn2+ metalloprotease with a characteristic inverted catalytic motif. IDE is ubiquitously expressed and degrades peptide substrates including insulin, endorphin, and the amyloid-beta peptide. Although IDE is mainly expressed in the cytosol, it can also be found on the cell surface and in secreted form in extracellular fluids. As IDE lacks a characteristic signal sequence that targets the protein to the classical secretory pathway, release of the enzyme involves non-conventional mechanisms. However, functional domains of IDE involved in its secretion remain elusive. By bioinformatical, biochemical, and cell biological methods, we identified a novel amino acid motif ((EKPPHY858)-E-853) close to the C terminus of IDE and characterized its function in the non-conventional secretion of the protein. Because of its close homology to an amino acid sequence found in bacterial proteins belonging to the SlyX family, we propose to call it the SlyX motif. Mutagenesis revealed that deletion of this motif strongly decreased the release of IDE, whereas deletion of a potential microbody-targeting signal at the extreme C terminus had little effect on secretion. The combined data indicate that the non-conventional secretion of IDE is regulated by the newly identified SlyX motif.