Detection and localization of synexin (Annexin VII) in Xenopus adult and embryonic tissues using an antibody to the Xenopus N-terminal PGQM repeat domain

Synexin (Annexin VII) is a widely distributed member of the annexin gene family which forms calcium channels and drives calcium-dependent membrane fusion. In Xenopus laevis, different synexins contain two to six tandem repeats of the tetra amino acid sequence PGQM in the unique N-terminal, with a distribution specific to adult tissues and embryonic stages. Immunogold studies using the PGQM-specific polyclonal antibody showed that synexin is localized in adult muscle to myosin-rich A-bands, Z-bands, and T-tubules, and in other adult tissues to nuclei and mitochondria and other formed elements. In oocytes, synexin was also found associated with yolk granules. The PGQM tandem repeats could represent interaction sites for other proteins, and we therefore synthesized a synthetic peptide containing the maximum six tandem repeats [NH~2-(PGQM)₆Y-COOH] to test this hypothesis. We found that the peptide alone could specifically bind and crosslink to different proteins in a tissue-specific manner. In liver, it bound to a single 35-kDa protein. In muscle, it bound to four proteins (35, 45, 48, and 116 kDa). Therefore, we conclude that the PGQM domain is accessible to specific antibodies and that the PGQM repeat is sufficiently ordered to unambiguously identify specific binding proteins in different Xenopus tissues.