Identification of different forms of calpastatin mRNA co-expressed in the notochord of Xenopus laevis embryos

We isolated three Xenopus cDNA clones, Xcalp1, Xcalp2 and Xcalp3, which encode different forms of calpastatin mRNA. Compared to the canonical form of mammalian calpastatin, the predicted Xcalp3 protein contained a very long N-terminal domain L and an additional inhibitory domain. The other two deduced calpastatin proteins were truncated forms, both lacking domain L and containing four (Xcalp2) and two (Xcalp1) inhibitory domains, respectively. The presence of Xcalp1, Xcalp2 and Xcalp3 transcripts was detected by in situ hybridization in the notochord from the embryonic stage 20 to stage 36, afterwards the expression was only present in the growing tailbud. As shown by RT-PCR, the three calpastatin mRNAs were also expressed in the adult brain.