The calpains: modular designs and functional diversity

Abstract

SummaryThe calpain family is named for the calcium dependence of the papain-like, thiol protease activity of the well-studied ubiquitous vertebrate enzymes calpain-1 (µ-calpain) and calpain-2 (m-calpain).Proteins showing sequence relatedness to the catalytic core domains of these enzymes are included in this ancient and diverse eukaryotic protein family. Calpains are examples of highly modular organization, with several varieties of amino-terminal or carboxy-terminal modules flanking a conserved core. Acquisition of the penta-EF-hand module involved in calcium binding (and the formation of heterodimers for some calpains) seems to be a relatively late event in calpain evolution. Several alternative mechanisms for binding calcium and associating with membranes/phospholipids are found throughout the family. The gene family is expanded in mammals, trypanosomes and ciliates, with up to 26 members in Tetrahymena, for example; in striking contrast to this, only a single calpain gene is present in many other protozoa and in plants. The many isoforms of calpain and their multiple splice variants complicate the discussion and analysis of the family, and challenge researchers to ascertain the relationships between calpain gene sequences, protein isoforms and their distinct or overlapping functions. In mammals and plants it is clear that a calpain plays an essential role in development. There is increasing evidence that ubiquitous calpains participate in a variety of signal transduction pathways and function in important cellular processes of life and death. In contrast to relatively promiscuous degradative proteases, calpains cleave only a restricted set of protein substrates and use complex substraterecognition mechanisms, involving primary and secondary structural features of target proteins. The detailed physiological significance of both proteolytically active calpains and those lacking key catalytic residues requires further study. Gene organization and evolutionary historyThis review focuses on the eukaryotic calpains, although genome databases reveal bacteria, but no archaea, with sequences related to the catalytic core domains (domains dI and dII) of the classical calpains, the criterion used for designating a protein as a calpain. Only single copies of calpain-coding genes are found in the small number of sequenced or partially sequenced protozoan genomes, such as those of the apicomplexan parasites Plasmodium falciparum, Theileria annulata and Cryptosporidium parvum [1][2][3], and of the amitochondrial parasite Entamoeba histolytica [4]. No calpain-like sequences were identified in the human pathogen Giardia lamblia, a diplomonad often considered to be the most basal eukaryotic organism [5]. Protozoan calpains lack a domain containing EF-hand-type Ca 2+ -binding sites, as also do plant and fungal calpains, and thus it seems likely that the proposed cysteine proteasecalmodulin gene fusion leading to the classical calpain structure (for earlier reviews see [6][7][8]) occurred exclusively within the animal lineage....

Augmented generation of protein fragments during wakefulness as the molecular cause of sleep: a hypothesis

“…To simplify discussion, only Ca 2þ transients and the resulting local increases of Ca 2þ are cited below as the cause of calpain activation. Note, however, that other calpain regulators, such as phospholipids, protein kinases, calpain-binding proteins, and the ability of some calpains to activate themselves and other calpains through specific cuts, are also involved in controlling calpains, [116][117][118][119][120] thereby shaping the in vivo repertoires of protein fragments generated by these proteases. Figure 1.…”

“…Calpain cleavage sites encompass $ 10 residues, involve both conformational and sequence determinants, and can be impaired or inactivated by missense mutations. [116][117][118][119][120]229 Caspase cleavage sites are simpler and can be readily inactivated by missense mutations. 179,230 Given the ease of elimination, on evolutionary timescales, of deleterious cleavage sites in neuronal and other proteins, it is striking, and most telling, that The first eight caspase-generated, proapoptotic Ct fragments are the recently examined and confirmed short-lived substrates of the Arg/N-end rule pathway.…”

“…Lys-calpain-2 is the Ct fragment of human calpain-2, an activated form of this calpain. [116][117][118][119][120] #34. Asncalpain-B is the calpain-generated Ct fragment of one of two major D. melanogaster calpains and an activated form of this calpain.…”

A Drosophila model identifies calpains as modulators of the human leukemogenic fusion protein AML1-ETO

“…We screened UAS-dsRNA transgenic lines targeting around 1,500 genes and recovered 8 candidates, among which a UAS-dsRNA line against calpainB (calpB) (28) that we studied in detail. Calpains are a large family of Ca 2ϩ -dependent proteases conserved throughout evolution (29). In humans, they consist of 14 members with ubiquitous or tissue A specific isoforms that influence many aspects of cell physiology such as cell migration, proliferation and apoptosis.…”

A calpain unique to alveolates is essential in Plasmodium falciparum and its knockdown reveals an involvement in pre-S-phase development

“…Calpains are found in organisms from bacteria to mammals and exhibit great divergence of sequence and domain structure but have homologous catalytic domains. Calpains have been implicated in diverse processes (3)(4)(5)(6), such as muscle function, cell signaling, migration and attachment, death, transformation, cell-cycle regulation, differentiation and development, and fungal alkali tolerance, although the precise physiological role of many of these calpains is still poorly understood (3,7).…”