Differential tissue expression of a calpastatin isoform in Xenopus embryos.

This study is aimed at demonstrating the role played by a calpastatin isoform (Xcalp3) in Xenopus embryos. A specific monoclonal antibody (mAb) was raised against a glutathione S-transferase (GST)-Xcalp3 fusion protein and characterized by immunoblotting and confocal fluorescence microscopy on stage 20-36 embryos. Under these conditions, calpastatin reactivity is associated with a major 110 kDa protein fraction and preferentially expressed by notochord and somitic cells. In notochord cells, anti-calpastatin reactive sites were initially restricted to the lurninal space of the vacuoles and later became diffused throughout the cytoplasm. In contrast, anti-calpastatin reactive sites in somitic cells were initially diffused throughout the cytoplasm and became restricted to a few intracellular granules in the later developmental stages.