Signal Peptide Peptidase-Type Proteases: Versatile Regulators with Functions Ranging from Limited Proteolysis to Protein Degradation.

Intramembrane proteases catalyze the unusual cleavage of peptide bonds in the plane of biological membranes. They are categorized according to their active site. The GxGD aspartyl proteases comprise presenilin, the signal peptide peptidase (SPP), and SPP-like (SPPL) proteases. Here we focus on the functionally related SPP and SPPL proteases, and review the current understanding of their substrate specificity and summarize known physiological functions in mammalian cells. We discuss how on the one hand regulated intramembrane proteolysis generates signaling molecules, and on the other hand how processes such as endoplasmic reticulum-associated degradation controls the quantity and activity of central regulators. While the enzymatic core of GxGD intramembrane proteases is conserved, association with regulatory factors and substrate adaptors may have tailored enzymes for various specific functions.

The Metastable XBP1u Transmembrane Domain Defines Determinants for Intramembrane Proteolysis by Signal Peptide Peptidase.

Unspliced XBP1 mRNA encodes XBP1u, the transcriptionally inert variant of the unfolded protein response (UPR) transcription factor XBP1s. XBP1u targets its mRNA-ribosome-nascent-chain-complex to the endoplasmic reticulum (ER) to facilitate UPR activation and prevents overactivation. Yet, its membrane association is controversial. Here, we use cell-free translocation and cellular assays to define a moderately hydrophobic stretch in XBP1u that is sufficient to mediate insertion into the ER membrane. Mutagenesis of this transmembrane (TM) region reveals residues that facilitate XBP1u turnover by an ER-associated degradation route that is dependent on signal peptide peptidase (SPP). Furthermore, the impact of these mutations on TM helix dynamics was assessed by residue-specific amide exchange kinetics, evaluated by a semi-automated algorithm. Based on our results, we suggest that SPP-catalyzed intramembrane proteolysis of TM helices is not only determined by their conformational flexibility, but also by side-chain interactions near the scissile peptide bond with the enzyme's active site.