MMP-21 was first described as an MMP in Xenopus (XMMP), expressed in early embryo development. Later, species orthologs were reported in the Newt Cynops, and in rat, mouse and human. A literature alias was initiated by the deposit of sequences for MMP-21/22, which were later renamed MMP-23, the cysteine-array MMP (CA-MMP). The mouse sequence seems also to be expressed early in embryo development, especially in the brain, and the human sequence has been reported in tumors. MMP-21 contains the canonical MMP elements of cysteine switch (PRCGVPD) and HExGHxxxxxH catalytic zinc site, without a transmembrane sequence. The Xenopus and Cynops sequences contain a potential furin-like cleavage site (RR/KKR), shortly after the cysteine switch, but the rat, mouse and human furin sites may not be functional. The hinge region is much shorter in MMP-21 than other MMPs, and the carboxyterminal half has some homology to vitronectin. Little is known about MMP-21 function, expression or distribution. The relative low homology with other MMPs suggests a different distribution, and the regulatory elements known suggest quite different promotion than “classical” MMPs. The Xenopus and Cynops sequences have two inserts relative to the human and murine sequences, one in the propeptide domain, and one right after the cysteine switch. Interestingly, the pI for the human and murine proteins is quite basic, 9.334, 9.801 and 9.294 respectively for human, rat and mouse, while the Xenopus and Cynops predicted pI are 7.674 and 7.416. The basic pI and vitronectin homology suggest ECM localization. The 569 amino acid human zymogen has a predicted mass of 65.01 kD, the mouse and rat sequences at 65.45 and 61.07 kD, and the Xenopus and Cynops both about 70 kD, without posttranslational modifications. A recommended starting concentration for Western blots is 1:1,000 when using colorimetric substrates such as BCIP/NBT, and 1:5,000 for chemiluminescent substrates. Higher concentrations of antibody may be needed for non-human samples.

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