Studies on transglutaminase activity during Drosophila oogenesis

In Drosophila melanogaster, the eggshell is composed of a vitelline membrane that is the innermost layer, a wax layer, a crystalline chorionic inner layer, an endochorion, and an exochorion which is the outer layer. A progressive cross-linking process that renders its components largely insoluble stabilizes the eggshell. The chorion becomes insoluble by the end of oogenesis by a peroxidase-type enzyme activity that cross-links tyrosine residues of chorion proteins. During the terminal stages of oogenesis the vitelline membrane proteins are bound by disulfide cross-linking. Successive unknown non-disulfide cross-linking renders the vitelline membrane completely insoluble when the egg moves through the oviducts and the uterus. Taking into account the aminoacids composition of the eggshell proteins, it has been suggested that transglutaminase-type enzyme my be involved in eggshell protein cross-linking. Transglutaminases (Tgases) are a widely distributed group of calcium-dependent enzymes mediating covalent cross-linking reactions between specific peptide-bound γ-glutamyl residues and various primary ε-amino groups of peptide-bound lysine or polyamines, acting as amine donor substrates. We have undertaken a study in order to assess if the Tgase is involved in Drosophila eggshell assembly. By using anti-guinea pig Tgase and anti-isopeptide-bond antibodies, and by analyzing the uptake of fluorescein cadaverine, we found Tgase activity during Drosophila oogenesis. More interesting, the data obtained by in vivo treatment with the Tgase inhibitor cystamine suggest that Tgase activity may be required for the proper egg chamber development and eggshell assembly.