NOVEL RETICULAR CALCIUM-BINDING PROTEIN IS PURIFIED ON TAIPOXIN COLUMNS

We identified, by affinity chromatography, two putative binding protei ns for the presynaptic snake venom toxin taipoxin, We have previously characterized one of these proteins [neuronal pentraxin (NP)] as a neu ronally secreted protein with homology to acute-phase proteins, Here w e report the identification of the second protein as a 49-kDa lumenal calcium binding protein that we have named taipoxin-associated calcium binding protein 49 (TCBP-49). This protein contains six EF-hand putat ive calcium binding domains and the carboxyl-terminal sequence His-Asp -Glu-Leu (HDEL), identical to the yeast endoplasmic reticulum retentio n signal. Message for this protein is present in brain, liver, muscle, heart, kidney, and testis. Antibodies to this protein label reticular organelles of neurons and glia. This localization and the specific en richment of native and recombinant TCBP-49 on columns of immobilized t aipoxin raise the possibility that this protein interacts with interna lized taipoxin, perhaps mediating its activation. The availability of pure TCBP-49 will allow direct tests of whether TCBP-49 alters the int egrity of the oligomeric structure, phospholipase activity, or toxicit y of taipoxin.