pH and water regulation in the silk gland of Bombyx mori in relation to gel-sol transition of the liquid silk in the glandular lumen

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The silk gland is a unique and indispensable tissue to produce the silk proteins (fibroin and sericin), making a cocoon. The gland secretes and temporarily stores the silk proteins within the glandular lumen without any denaturation of proteins till the larva starts to spin. The concentrations in the secreted protein reach up to 20-30 % levels. Such a high concentration of the protein solution has to be regulated under a certain pH ranges and a fixed water content. We have investigated such a mechanism to understand the ionic homeostasis and osmoregulation in the gland. Results are as follows :(1) Immunocytochemical studies were done with the specific anitibody against V-ATPase. The V-ATPase can be detected in the middle silk gland cell(MSG) as well as in the anterior silk gland (ASG) cell. V-ATPase was located at the cell surface, suggesting that this V-ATPase is a plasma membrane-type not an endomembrane-type such as the V-ATPase in lysosomes and secretory vesicles.(2) The similar plasma membrane V-ATPase detected in another cocoon-producing lepidopteran insect, Eri-silkworm (Samia cynthia ricini), but not in the labial gland from the sweet potato hornworm, Agrius convolvuli, where we have detected the endomembrane V-ATPase in the cytoplasm. Thus, the presence of plasma membrane V-ATPase is a unique feature in the silk gland of lepidoptera.(3) By the homology cloning strategy, we have patially cloned the putative water channel (aquaporin, AQP) in the silk gland of Bombyx mori. This putative AQP is highly expressed in the ASG at the feeding stage of the 5th instar. The expression of the putative AQP seems to be stopped at the onset of spinning. Thus, it is suggested that the AQP as well as V-ATPase is closely related to the silk gland development and the cocoon production during larval-pupal metamorphosis.