Signal Peptide Peptidase (Recombinant)

Frequently Asked Questions

What molecular weight does recombinant Signal Peptide Peptidase run at on SDS-PAGE?

The SPP polypeptide is 377 amino acids with a predicted unmodified MW of ~42 kDa. Because it is expressed in HEK293 cells, N-linked glycosylation and membrane-topology-driven anomalous SDS binding typically shift the apparent MW to ~45–50 kDa on reducing SDS-PAGE. If you are using this protein as a loading control or positive-control band, anticipate the smeared or diffuse migration pattern common to multi-pass integral membrane proteins solubilized in standard SDS-PAGE conditions. Running under non-reducing conditions may sharpen the band slightly.

Which isoform or processing form of SPP does this recombinant represent?

This recombinant corresponds to the full-length, catalytically active form of human SPP (HM13, UniProt Q8TCT9), encompassing all 377 amino acids including both catalytic aspartate-containing GxGD motifs. It is not a truncated or signal-peptide-cleaved fragment — the HEK293 expression system retains the native membrane topology during production. No propeptide processing event equivalent to classical aspartyl protease activation is documented for SPP; the full-length form is the active enzyme.

What substrates does Signal Peptide Peptidase cleave and how do I run an in vitro activity assay?

SPP cleaves signal peptides within the ER membrane after their Sec61 translocase-mediated removal. Well-characterized in vitro substrates include the MHC class I HLA-A2 signal peptide (used to generate the HLA-E nonapeptide epitope) and the hepatitis C virus core protein signal peptide. For fluorogenic activity assays, membrane-mimetic conditions are essential: reconstitute the reaction in detergent-containing buffer (0.1–0.5% CHAPSO or DDM work well in published SPP assays) supplemented with our standard buffer (50 mM Tris-HCl pH 7.5, 150 mM NaCl). FRET-based peptide substrates flanked with EDANS/DABCYL can be adapted from presenilin γ-secretase assay formats with SPP-specific cleavage site sequences.

What starting concentration of recombinant SPP should I use for an inhibitor IC50 assay?

A good starting point is 0.5–2 nM recombinant SPP per reaction well, titrating down if background hydrolysis of the fluorogenic substrate is too high. Because SPP is an intramembrane protease, detergent micelle composition substantially affects apparent Km and thus IC50 values — standardize your detergent lot and concentration before comparing inhibitor series. Known SPP inhibitors such as (Z-LL)2-ketone and helenalin serve as useful reference compounds to validate assay window (Z-score ≥ 0.5) before screening. Pre-incubate enzyme with inhibitor for 30 minutes at 37°C before substrate addition.

What buffer conditions are optimal for storing and diluting recombinant Signal Peptide Peptidase?

The protein is supplied in 50 mM Tris-HCl pH 7.5, 150 mM NaCl, 10% glycerol, which stabilizes the detergent-solubilized membrane protein during storage. Store at -20°C in single-use aliquots; repeated freeze-thaw cycles cause aggregation and loss of activity. For working dilutions, use the same buffer supplemented with 0.05–0.1% DDM or CHAPSO to prevent hydrophobic aggregation at sub-micromolar concentrations. Avoid dilution into detergent-free aqueous buffers — SPP rapidly aggregates without a membrane-mimetic environment. Prepare dilutions fresh on the day of use and keep on ice.

Can I use this recombinant SPP as a positive control for Western blot with the matched anti-SPP antibody?

Yes — this recombinant is validated as a positive control for the matched rabbit polyclonal antibody (SKU: RP-Signal Peptide Peptidase; /anti-signal-peptide-peptidase-rabbit-polyclonal-antibody), which was raised and tested in the same laboratory. Both reagents were developed together specifically to guarantee this compatibility. Load 20–50 ng of recombinant SPP per lane on a standard 12% SDS-PAGE gel alongside your cell lysate samples. The antibody detects the ~45–50 kDa glycosylated band under reducing conditions, confirming antibody specificity before you commit to a full experimental blot.

How much recombinant Signal Peptide Peptidase should I load as a positive control for Western blot?

20–50 ng per lane is the recommended loading range when using the matched antibody (RP-Signal Peptide Peptidase). At 20 ng you should see a clean band without overwhelming adjacent lysate lanes; 50 ng is appropriate if you are running a sensitivity titration or validating a new secondary antibody. Dilute the stock in 1× Laemmli sample buffer containing 5% β-mercaptoethanol and heat at 70°C (not 95°C) for 5 minutes — harsh boiling of membrane proteins promotes aggregation and produces smeared or lost bands on transfer.

How does HEK293 expression preserve SPP activity compared to E. coli or baculovirus systems?

SPP is a 7-transmembrane aspartyl protease that absolutely requires correct membrane integration and mammalian-type glycosylation for catalytic competence. E. coli lacks the ER translocation machinery needed to insert multi-pass membrane proteins in the correct topology, typically yielding inclusion bodies. Baculovirus-insect cell systems can insert membrane proteins but provide partially truncated N-glycan structures that affect protein stability and folding. HEK293 expression replicates the mammalian ER environment, supporting native GxGD active-site geometry and full glycosylation, which is why this recombinant retains measurable intramembrane protease activity in detergent-reconstituted assays.