fc177, a minor dec-1 proprotein, is necessary to prevent ectopic aggregation of the endochorion during eggshell assembly in Drosophila.

The Drosophila eggshell is a highly specialized extracellular matrix that forms between the oocyte and the surrounding epithelial follicle cells during late oogenesis. The dec-1 gene, which is required for proper eggshell assembly, produces three proproteins that are cleaved within the vitelline membrane layer to multiple derivatives. The different spatial distributions of the cleaved derivatives suggest that they play distinct roles in eggshell assembly. Using extant dec-1 mutations in conjunction with genetically engineered dec-1 transgenes, we show that, although all three dec-1 proproteins, fc106, fc125, and fc177, are required for female fertility, gross morphological abnormalities in the eggshell are observed only in the absence of fc177. The coalescence of the roof, pillar, and floor substructures of the tripartite endochorion suggested that quantitatively minor fc177 derivatives are necessary to prevent ectopic aggregation of endochorion proteins during the assembly process. Expression of a fc177 cDNA in dec-1 null mutants was sufficient to restore spaces within the endochorion layer. Fc177 may function as a scaffolding protein akin to those utilized in viral morphogenesis.