Abstract
Calpains (CAPN) are a family of Ca2+-dependent cysteine proteases that regulate various cellular functions by cleaving diverse substrates. Of the 15 mammalian calpains, CAPN8 and CAPN9 are two that are expressed predominantly in the gastrointestinal tract, where they interact to form a protease complex, termed G-calpain. However, because native G-calpain exhibits a highly restricted expression pattern, it has never been purified, and the interactions between CAPN8 and CAPN9 have not been characterized. Here, we clarified the molecular nature of G-calpain by using recombinant proteins and transgenic mice expressing FLAG-tagged CAPN8 (CAPN8-FLAG). Recombinant mouse CAPN8 and CAPN9 co-expressed in eukaryotic expression systems exhibited the same mobility as native mouse G-calpain in Blue Native-PAGE gels, and CAPN8-FLAG immunoprecipitation from stomach homogenates of the transgenic mice showed that CAPN9 was the only protein that associated with CAPN8-FLAG. These results indicated that G-calpain is a heterodimer of CAPN8 and CAPN9. In addition, active recombinant G-calpain was expressed and purified using an in vitro translation system, and the purified protease exhibited enzymatic properties that were comparable with that of calpain-2. We found that an active-site mutant of CAPN8, but not CAPN9, compromised G-calpain's substrate cleavage activity, and that the N-terminal helix region of CAPN8 and the C-terminal EF-hands of CAPN8 and CAPN9 were involved in CAPN8/9 dimerization. Furthermore, CAPN8 protein in Capn9-/- mice was almost completely lost, whereas CAPN9 was only partially lost in Capn8-/- mice. Collectively, these results demonstrated that CAPN8 and CAPN9 function as catalytic and chaperone-like subunits, respectively, in G-calpain.
Highlights
Calpains (CAPN) are a family of Ca2؉-dependent cysteine proteases that regulate various cellular functions by cleaving diverse substrates
Recombinant mouse CAPN8 and CAPN9 co-expressed in eukaryotic expression systems exhibited the same mobility as native mouse G-calpain in Blue Native-PAGE gels, and CAPN8-FLAG immunoprecipitation from stomach homogenates of the transgenic mice showed that CAPN9 was the only protein that associated with CAPN8-FLAG
Our results showed that G-calpain is a heterodimer of CAPN8 and CAPN9 that requires the proteolytic activity of CAPN8 to cleave substrates
Summary
CysPc, calpain-type Cys protease conserved domain; PC, protease core domain; PEF, penta-EF-hand domain; NS, N-terminal sequence; IS, internal sequence; BN-PAGE, Blue-Native PAGE; Tg, transgenic; CBB, Coomassie Brilliant Blue; Suc, succinyl; MCA, 4-methylcoumaryl-7-amide; Edans, 5-[(2-aminoethyl) amino]naphthalene-1-sulfonic acid; (Dabcyl)-R, 4-((4-(diethylamino)phenyl)azo)benzoic acid-Arg; Cre, causes recombination; aa, amino acids; FL, full-length. The binding of Ca2ϩ to the PC1 and PC2 domains induces the active conformation [21,22,23]. This activation process occurs concomitantly with the autolysis of the N-terminal anchor region and the Gly-rich domain (GR) of CAPNS1 [24, 25]. We recently demonstrated that CAPN8 and CAPN9 form an active protease complex, termed G-calpain, in which both are essential for its function, and that they play physiological roles in stress-induced gastric mucosal defense [15]. We sought to clarify the composition of G-calpain’s subunits and the molecular mechanism underlying their interactions, which may shed light on how G-calpain is regulated for gastric mucosal defense. Our results showed that G-calpain is a heterodimer of CAPN8 and CAPN9 that requires the proteolytic activity of CAPN8 to cleave substrates
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