Bmpallidin, encoding a subunit of the BLOC-1 complex, is involved in urate granules formation in silkworm integument

The abnormal development of urate granules in silkworm larvae leads to translucent mutants with a distinct transparent phenotype. Studies on such mutants are expected to enhance current understanding of uric acid metabolism. The hoarfrost translucent (oh) mutant exhibits a mottled, translucent larval integument due to the presence of smaller and irregularly shaped urate granules compared to wild-type individuals. Uric acid content in the silkworm larval integuments is significantly lower in the oh mutant. Using positional cloning, we successfully narrowed a ~ 180 kb region linked to the oh locus and identified the candidate gene, Bmpallidin, encoding the biosynthesis of lysosome-related organelles complex 1 subunit 6. Three alternative splicing isoforms were identified; Only isoform II was predicted to translate normally and was drastically reduced at both mRNA and protein levels in the oh mutant. Conversely, the non-functional isoform III showed slightly increased expression in the mutant. An 860 bp genomic sequence in the wild type was replaced by a 30 bp sequence in the mutant. Knockdown of Bmpallidin induced a translucent phenotype in first-instar larvae. These findings conclude that Bmpallidin is responsible for the oh mutant phenotype and plays a crucial role in urate granules formation in silkworms.