Relationship between Rab and insulin-like proteins in the nervous system of Bombyx mori

Rab proteins are small GTP-binding proteins and are the largest family in the Ras GTPase superfamily and mediate vesicular transport in cells. Diverse insulin-like peptides, such as bombyxin, are synthesized in the brain and secreted into the haemolymph by the corpus allatum (CA). In the brain of Bombyx mori, Rabs are expressed in a specific area; however, which Rabs actually link the secretion of bombyxin remains unknown. A double-staining analysis of nine Rabs ( Rab1, 3, 6, 7, 14, 21, 26, 39 and X4) and bombyxin indicated that Rab3-, Rab7-, Rab39-and RabX4-immunohistochemical reactivity (ir) areas overlapped with bombyxin-ir in the brain and CA in B. mori, while Rab6-, Rab14-and Rab21-irs partially overlapped in the CA. Rab1-ir occurred in the other immunopositive areas in CA. Rab26-ir did not occur in the brain. Rab39-ir occurred in UNC104, Rab39- effector,-immunopositive neurons in the brain and CA. Thus, Rab3, 7, 39 and X4 may regulate the exocytosis of bombyxin.