Structural studies on the major milk gland protein of the tsetse fly, Glossina morsitans morsitans.

1. The major protein in the milk gland secretions of the tsetse fly, Glossina morsitans morsitans, was isolated by a combination of gel permeation chromatography and crystallization. 2. It has a native Mr approximately 47,000 and is composed of two identical polypeptide chains (Mr approximately 21,000) as determined by chemical cross-linking studies. The protein has no covalently-bound carbohydrates or lipids. Amino acid analysis of the protein revealed relatively high amounts of the aromatic amino acids, tyrosine (9.1 mol.%) and phenylalanine (8.5 mol.%). Immunoblotting experiments using antiserum against the protein revealed no cross-reactivity with any other milk proteins. 3. Quantitation of the protein during the pregnancy cycle showed that synthesis of the protein by the milk glands of adult female flies starts as the larva moults into second instar and rapidly declines as it matures into third instar. 4. It is proposed that the major milk gland protein could provide essential amino acids needed for the puparium formation.