Proximity ligation assays of protein and RNA interactions in the male-specific lethal complex on Drosophila melanogaster polytene chromosomes.

In Drosophila, the male-specific lethal (MSL) complex specifically targets the male X chromosome and participates in a twofold increase in expression output leading to functional dosage compensation. The complex includes five proteins and two non-coding RNAs (ncRNAs). A number of additional associated factors have also been identified. However, the components' roles and interactions have not been fully elucidated. The in situ proximity ligation assay (PLA) provides a sensitive means to determine whether proteins and other factors have bound to chromosomes in close proximity to each other, and thus may interact. Thus, we modified, tested, and applied the assay to probe interactions of MSL complex components on polytene chromosomes. We show that in situ PLA can detect and map both protein-protein and protein-ncRNA interactions on polytene chromosomes at high resolution. We further show that all five protein components of the MSL complex are in close proximity to each other, and the ncRNAs roX1 and roX2 bind the complex in close proximity to MLE. Our results also indicate that JIL1, a histone H3 Ser10 kinase enriched on the male X chromosome, interacts with MSL1 and MSL2, but not MSL3 of the MSL complex. In addition, we corroborate proposed interactions of the MSL complex with both CLAMP and TopoII.