CASPASE: STRUCTURE, TYPES AND FUNCTIONS - BIOLOGY - 2025

The caspases are effector proteins route of programmed cell death or apoptosis. They belong to a family of highly conserved cysteine-dependent and aspartate-specific proteases, from which their name comes.

They employ a cysteine ​​residue in their active site as a catalytic nucleophile to cleave protein substrates with aspartic acid residues in their structures and this function is crucial for the execution of the apoptotic program.

Structure of caspase-3 (Source: Jawahar Swaminathan and MSD staff at the European Bioinformatics Institute via Wikimedia Commons)

Apoptosis is an extremely important event in multicellular organisms, since it plays an important role in the maintenance of homeostasis and tissue integrity.

The role of caspases in apoptosis contributes to the critical processes of homeostasis and repair, as well as the cleavage of structural components that result in the orderly and systematic dismantling of the dying cell.

These enzymes were first described in C. elegans and then related genes were found in mammals, where their functions were established through different genetic and biochemical approaches.

Structure

Each active caspase derives from the processing and self-association of two precursor zymogenic pro-caspases. These precursors are tripartite molecules with "dormant" catalytic activity and a molecular weight ranging from 32 to 55 kDa.

The three regions are known as p20 (large internal central domain of 17-21 kDa and containing the active site of the catalytic subunit), p10 (C-terminal domain of 10-13 kDa also known as small catalytic subunit) and DD domain. (death domain, 3-24 kDa, located at the N-terminus).

In some pro-caspases the p20 and p10 domains are separated by a small sequence of spacing. The prodomains of death or DD at the N-terminal end have 80-100 residues that constitute the structural motifs of the superfamily involved in the transduction of apoptotic signals.

The DD domain, in turn, is divided into two sub-domains: the effector death domain (DED) and the caspase recruitment domain (CARD), which are made up of 6-7 antiparallel α-amphipathic helices that interact with other proteins through electrostatic or hydrophobic interactions.

Caspases possess many conserved residues that are responsible for the general establishment of structure and their interaction with ligands during assembly and processing of zymogens, as well as with other regulatory proteins.

Pro-caspases 8 and 10 possess two DED domains arranged in tandem within their pro-domain. Pro-caspases 1, 2, 4, 5, 9, 11, and 12 possess a CARD domain. Both domains are responsible for the recruitment of the initiator caspases to the complexes that induce death or inflammation.

Activation

Each pro-caspase is activated by responding to specific signals and by selective proteolytic processing at specific aspartic acid residues. Processing ends with the formation of homodimeric proteases that initiate the apoptotic process.

The initiator caspases are activated by dimerization, while the effector ones are activated by cleavage of the inter-domains. There are two routes for the activation of caspases; the extrinsic and the intrinsic.

The extrinsic pathway or death receptor-mediated pathway implies the participation of the death signaling complex as an activator complex for pro-caspases-8 and 10.

The intrinsic pathway or the mitochondrial-mediated pathway employs the apoptosome as an activator complex for pro-caspase-9.

Types

Mammals have around 15 different caspases, coming from the same genetic family. This superfamily encompasses other subfamilies that are categorized depending on the position of the pro-domains and their functions.

Typically 3 subclasses of caspases are known in mammals:

1-Inflammatory or group I caspases: caspases with large pro-domains (Caspase-1, caspase-4, caspase-5, caspase-12, caspase-13 and caspase-14) that have a fundamental role in the maturation of cytokines and in the inflammatory response.

2-Apoptosis initiating or group II caspases: they have a long pro-domain (more than 90 amino acids) containing either a DED domain (caspase-8 and caspase-10) or a caspase recruitment domain (caspase-2 and caspase-9)

3-Effector caspases or group III: they have short pro-domains (20-30 amino acids).

Features

Most of the functions of individual caspases have been elucidated by means of genetic silencing experiments or obtaining mutants, establishing particular functions for each one.

Apoptotic functions

Although there are caspase-independent apoptotic pathways, these enzymes are critical for many of the programmed cell death events, necessary for the correct development of most of the systems of multicellular organisms.

In apoptotic processes, the initiating caspases are caspases -2, -8, -9 and -10, while among the effector caspases are caspases -3, -6 and -7.

Its specific intracellular targets include nuclear lamina and cytoskeletal proteins, the cleavage of which promotes cell death.

Non-apoptotic functions

Caspases not only play an apoptotic role in the cell, since the activation of some of these enzymes has been demonstrated in the absence of cell death processes. Its non-apoptotic role involves proteolytic and non-proteolytic functions.

They participate in the proteolytic processing of enzymes in order to avoid cell dismantling; its targets include proteins such as cytokines, kinases, transcription factors, and polymerases.

These functions are possible thanks to the post-translational processing of pro-caspases or their proteolytic targets, to the spatial separation of enzymes between cell compartments, or to regulation by other upstream effector proteins.

Immune function

Some caspases are involved in the processing of important factors in the immune system, such is the case of caspase-1, which processes pro-Interleukin-1β to form mature IL-1β, which is a key mediator for the inflammatory response.

Caspase-1 is also responsible for the processing of other interleukins such as IL-18 and IL-33, which participate in the inflammatory response and in the innate immune response.

In cell proliferation

In many ways, caspases are involved in cell proliferation, especially in lymphocytes and other cells of the immune system, with caspase-8 being one of the most important enzymes involved.

Caspase-3 also appears to have functions in cell cycle regulation, since it is capable of processing the cyclin-dependent kinase (CDK) inhibitor p27, which contributes to the progression of cell cycle induction.

Other functions

Some caspases participate in the progress of cell differentiation, especially of cells entering a post-mitotic state, which is sometimes considered as a process of incomplete apoptosis.

Caspase-3 is critical for the proper differentiation of muscle cells, and other caspases are also involved in the differentiation of myeloids, monocytes, and erythrocytes.

References

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