Localization of a putative inositol 1,4,5-triphosphate receptor in the Limulus granulocyte.

The horseshoe crab (Limulus polyphemus) granulocyte (GR) degranulates upon contact with bacteria and release factors that mediate an immune response. Stimulated cells produce IP3, which binds to receptors (IP3R, M.W.240-300 kD) that function to release stored Ca2+ into the cytoplasm that mediates degranulation. This mechanism is believed to mediate exocytosis in the Limulus GR but IP3R in the GR has not been shown. The present study utilized monoclonal antibody 4C11 and a commercially available anti-IP3R antibody, both of which label amino acids of the N-terminal of all known isoforms. Electron microscopy, immunohistochemistry, SDS-PAGE, and Western blot analysis, which employed the use of the two antibodies, demonstrates that a putative IP3R exists in the: plasma membrane, smooth surfaced vesicles, nucleus and nuclear membrane. We hypothesize that this putative IP3R is involved in mediating the immune response of the Limulus GR.