Catalytic activity of the Bin3/MEPCE methyltransferase domain is dispensable for 7SK snRNP function in .

Methylphosphate Capping Enzyme (MEPCE) monomethylates the gamma phosphate at the 5' end of the 7SK noncoding RNA, a modification thought to protect 7SK from degradation. 7SK serves as a scaffold for assembly of a snRNP complex that inhibits transcription by sequestering the positive elongation factor P-TEFb. While much is known about the biochemical activity of MEPCE , little is known about its functions , or what roles- if any-there are for regions outside the conserved methyltransferase domain. Here, we investigated the role of Bin3, the ortholog of MEPCE, and its conserved functional domains in development. We found that mutant females had strongly reduced rates of egg-laying, which was rescued by genetic reduction of P-TEFb activity, suggesting that Bin3 promotes fecundity by repressing P-TEFb. mutants also exhibited neuromuscular defects, analogous to a patient with haploinsufficiency. These defects were also rescued by genetic reduction of P-TEFb activity, suggesting that Bin3 and MEPCE have conserved roles in promoting neuromuscular function by repressing P-TEFb. Unexpectedly, we found that a Bin3 catalytic mutant (Bin3) could still bind and stabilize 7SK and rescue all mutant phenotypes, indicating that Bin3 catalytic activity is dispensable for 7SK stability and snRNP function . Finally, we identified a metazoan-specific motif (MSM) outside of the methyltransferase domain and generated mutant flies lacking this motif (Bin3). Bin3 mutant flies exhibited some-but not all- mutant phenotypes, suggesting that the MSM is required for a 7SK-independent, tissue-specific function of Bin3.