Dipeptidyl peptidase activity masks caspase cleavage sites, revealing hidden substrates in apoptotic cells
Background
Caspases are crucial cysteine proteases that regulate diverse cellular processes, including apoptosis, inflammation, and differentiation, by cleaving specific target proteins. Identifying these cleavage events and their precise sites is vital for understanding cell fate decisions. Proteomic techniques like N-terminomics have been instrumental in this, yet some caspase substrates remain elusive. The current challenge lies in fully capturing the landscape of caspase-mediated proteolysis, especially when secondary proteolytic events might obscure primary cleavage sites, leading to an incomplete picture of cellular signaling pathways during programmed cell death.
Study Design
Researchers re-analyzed a previously generated N-terminomics dataset from differentiating and dying C2C12 myoblasts. The study focused on identifying caspase substrates and cleavage sites by comparing protein N-termini profiles between cells undergoing apoptosis and those undergoing differentiation. This involved a detailed computational analysis of peptide sequences derived from the neo-N-termini, aiming to uncover patterns indicative of secondary proteolytic processing that might obscure initial caspase activity. The methodology specifically looked for modifications to the expected caspase cleavage motifs.
Results
The re-analysis revealed a novel mechanism where caspase-mediated cleavage events are 'hidden' by subsequent proteolytic activity. They propose that after a caspase cleaves a protein, a dipeptidyl peptidase (DPP) removes two amino acids from the newly formed N-terminus. This secondary cleavage event alters the peptide sequence that N-terminomics typically uses for identification, thereby masking the original caspase cleavage site. This mechanism means that existing N-terminomics datasets likely contain numerous unidentified caspase substrates. Importantly, this 'hidden' cleavage of substrates was specifically observed in apoptotic myoblasts but was absent in differentiating myoblasts, even though both cell types contained active caspase-3. This suggests the DPP-mediated trimming is a specific feature of the apoptotic pathway.
This secondary processing by DPPs after caspase cleavage provides a new layer of complexity to understanding cell fate decisions and expands the known repertoire of caspase substrates.
Key Findings
- Caspase cleavage sites can be masked by secondary proteolysis from dipeptidyl peptidases (DPPs).
- DPPs remove two amino acids from neo-N-termini after caspase cleavage, hiding the original site.
- This 'hidden' cleavage mechanism was found in apoptotic C2C12 myoblasts.
- The DPP-mediated trimming was absent in differentiating myoblasts, suggesting specificity to apoptosis.
- Existing
N-terminomicsdatasets likely contain many hitherto unidentified caspase substrates.
Why It Matters
This discovery significantly impacts the interpretation of N-terminomics data, enabling the identification of previously overlooked caspase substrates. For researchers and biohackers, this means a more comprehensive understanding of apoptosis and other cell fate decisions, potentially revealing new targets for therapeutic intervention. Understanding this interplay between caspases and DPPs could lead to novel biomarkers for detecting apoptotic cells or specific disease states where programmed cell death is dysregulated. While this is a preclinical in-vitro finding, it lays foundational knowledge that could inform future drug discovery efforts targeting specific proteolytic pathways, potentially leading to more precise control over cell survival and death.
apoptosis
caspases
dipeptidyl peptidase
n-terminomics
cell death
proteomics