RNA provides the framework for the assembly of some of the most intricate 
macromolecular complexes within the cell, including the spliceosome and the 
mature ribosome. The assembly of these complexes relies on the coordinated 
association of RNA with hundreds of trans-acting protein factors. While some of 
these trans-acting factors are RNA-binding proteins (RBPs), others are adaptor 
proteins, and others still, function as both. Defects in the assembly of these 
complexes results in a number of human pathologies including neurodegeneration 
and cancer. Here, we demonstrate that Silencing Defective 2 (SDE2) is both an 
RNA binding protein and also a trans-acting adaptor protein that functions to 
regulate RNA splicing and ribosome biogenesis. SDE2 depletion leads to 
widespread changes in alternative splicing, defects in ribosome biogenesis and 
ultimately complete loss of cell viability. Our data highlight SDE2 as a 
previously uncharacterized essential gene required for the assembly and 
maturation of the complexes that carry out two of the most fundamental processes 
in mammalian cells.