COMPLEX-FORMATION BY THE DROSOPHILA MSL PROTEINS - ROLE OF THE MSL2 RING FINGER IN PROTEIN COMPLEX ASSEMBLY

Drosophila MSL proteins are thought to act within a complex to elevate transcription from the male X chromosome. We found that the MSL1, MSL 2 and MSL3 proteins are associated in immunoprecipitations, chromatogr aphic steps and in the yeast two-hybrid system, but that the MLE prote in is not tightly complexed in these assays. We focused our analysis o n the MSL2-MSL1 interaction, which is postulated to play a critical ro le in MSL complex association with the X chromosome. Using a modified two-hybrid assay, we isolated missense mutations in MSL2 that disrupt its interaction with MSL1. Eleven out of 12 mutated residues clustered around the first zinc-binding site of the RING finger domain were con served in a Drosophila virilis MSL2 homolog. Two pre-existing msl2 all eles, which fail to support male viability in vivo, have lesions in th e same region of the RING finger. We tested these in the two-hybrid sy stem and found that they are also defective in interaction with MSL1, Mutation of the second zinc-binding site had little effect on MSL1 bin ding, suggesting that this portion of the RING finger may have a disti nct function. Our data support a model in which MSL2-MSL1 interaction nucleates assembly of an MSL complex, with which MLE is weakly or tran siently associated.