Drosophila Amus and Bin3 methylases functionally replace mammalian MePCE for capping and the stabilization of U6 and 7SK snRNAs

U6 and 7SK snRNAs process a 5 cap, believed to be essential for their stability and maintained by mammalian MePCE or Drosophila Bin3 enzymes. Although loss of either protein results in 7SK instability, loss of neither is associated with U6 reduction. Their yeast homolog is also not required for U6 stability, casting further doubts on the function of capping U6. Here we show that the Drosophila Amus protein, homologous to both Bin3 and MePCE, is essential for U6 but not 7SK stability. A full function of Amus is required for Drosophila development, and that rests primarily on Amuss methylase activity. Remarkably, the loss of U6 is rescued by the expression of an Amus-MePCE hybrid protein harboring the methyltransferase domain from MePCE, highlighting the conserved function of the two proteins as the U6 capping enzyme. Our new investigations in human cells establish a dependence of both U6 and 7SK stability on MePCE, resolving a long-standing uncertainty. While uncovering an interesting division of labor of Bin3/MePCE/Amus proteins, we discovered a "Bin3-Box" domain present only in enzymes associated with 7SK regulation. Targeted mutagenesis in Drosophila confirmed its importance for Bin3 function, revealing a possible conserved element in 7SK but not U6 biology.