GO:0004298 threonine-type endopeptidase activity

Threonine-type endopeptidase activity (GO:0004298) is a molecular function defined by a unique catalytic strategy: the hydroxyl group of a threonine residue at the active site acts as a nucleophile to cleave internal peptide bonds in polypeptide chains. This mechanism is distinct from the more common serine, cysteine, or aspartic proteases and is best exemplified by the proteasome, a large multicatalytic protease complex that degrades ubiquitinated proteins. The term encompasses several synonymous activities, including 26S protease, multicatalytic proteinase, and prosome, reflecting its historical discovery in diverse contexts. Understanding this activity is fundamental for researchers in cell biology, oncology, and neurodegeneration because it controls protein turnover, antigen presentation, and stress responses. Recent proteomic analyses in sheep have also highlighted genotype-specific expression of proteins with this activity, suggesting roles in reproduction and development. As a result, GO:0004298 is a key annotation for interpreting gene function and for designing experiments that manipulate protein degradation.

In simple terms, threonine-type endopeptidase activity is the ability of an enzyme to cut other proteins internally using a threonine amino acid as the chemical tool. According to QuickGO, it is defined as catalysis of the hydrolysis of internal peptide bonds in a polypeptide chain by a mechanism in which the hydroxyl group of a threonine residue at the active center acts as a nucleophile. This activity is classified under molecular_function and is often associated with the proteasome endopeptidase complex, also known as the 26S protease or multicatalytic proteinase.

Threonine-type endopeptidase activity is essential for controlled protein degradation, which regulates virtually every cellular process, including cell cycle progression, apoptosis, and immune response. Because the proteasome is the primary executor of this activity, its dysfunction or overactivity contributes to cancer, neurodegenerative diseases, and inflammatory conditions. Small-molecule inhibitors of this activity are used both as research tools and as anticancer drugs, underscoring its translational relevance. Moreover, proteomic studies in livestock have linked this activity to economically important traits such as fertility, indicating broader biological significance.

The following genes encode proteins that either possess threonine-type endopeptidase activity or are essential for its regulation and assembly.

Threonine-type endopeptidase activity is regulated at multiple levels. Transcription of proteasome subunit genes is controlled by transcription factors such as Nrf1 and Nrf2 in response to proteotoxic stress. Post-translational modifications, including phosphorylation and ubiquitination, modulate proteasome assembly and activity. Additionally, the activity is inhibited by endogenous proteins such as PI31 and by small-molecule inhibitors that target the active site threonine. In sheep, proteomic analysis revealed that proteins annotated with this activity differ among FecB genotypes, suggesting genetic regulation.

Proteasome inhibitors that target threonine-type endopeptidase activity, such as bortezomib, are used to treat multiple myeloma and mantle cell lymphoma. Cancer cells are more sensitive to proteasome inhibition due to their high protein turnover and dependence on NF-kB signaling.

Impaired proteasome activity contributes to the accumulation of misfolded proteins in Alzheimer's and Parkinson's diseases. Enhancing or restoring threonine-type endopeptidase activity is a potential therapeutic strategy.

Immunoproteasome subunits (PSMB8, PSMB9, PSMB10) are induced by interferon and are involved in antigen presentation; their dysregulation is linked to autoimmune diseases.

Proteomic differences in sheep with different FecB genotypes include proteins with threonine-type endopeptidase activity, suggesting a role in fertility.

Fluorogenic peptide substrates specific for chymotrypsin-like, trypsin-like, and caspase-like activities are used to measure threonine-type endopeptidase activity in cell lysates or purified proteasomes.

Chemical probes that covalently label the active site threonine allow visualization and quantification of active proteasome complexes in gels or live cells.

Mass spectrometry-based proteomics can identify proteins with this activity and quantify changes in expression across genotypes or treatments, as shown in sheep semen analysis.

Genome-wide CRISPR knockout or activation screens can identify genes that regulate threonine-type endopeptidase activity or confer resistance to inhibitors.

Researchers studying threonine-type endopeptidase activity-related genes often need to determine whether a candidate gene is causally involved in proteasome function, drug response, or disease. EDITGENE provides a comprehensive suite of CRISPR services to generate precisely engineered cell models that answer these questions.

Threonine-type endopeptidase activity (GO:0004298) is a fundamental molecular function that governs protein degradation through a unique catalytic mechanism. Its central role in the proteasome makes it a critical target for cancer therapy and a key player in neurodegeneration and immune responses. Recent proteomic studies in livestock further highlight its broader biological significance. By leveraging CRISPR-based models and advanced proteomic methods, researchers can continue to unravel the complexities of this activity and develop new therapeutic strategies.