Three RNA Binding Proteins Form a Complex to Promote Differentiation of Germline Stem Cell Lineage in <i>Drosophila</i>

<div><p>In regenerative tissues, one of the strategies to protect stem cells from genetic aberrations, potentially caused by frequent cell division, is to transiently expand the stem cell daughters before further differentiation. However, failure to exit the transit amplification may lead to overgrowth, and the molecular mechanism governing this regulation remains vague. In a <i>Drosophila</i> mutagenesis screen for factors involved in the regulation of germline stem cell (GSC) lineage, we isolated a mutation in the gene <i>CG32364</i>, which encodes a putative RNA-binding protein (RBP) and is designated as <i>tumorous testis</i> (<i>tut</i>). In <i>tut</i> mutant, spermatogonia fail to differentiate and over-amplify, a phenotype similar to that in <i>mei-P26</i> mutant. Mei-P26 is a TRIM-NHL tumor suppressor homolog required for the differentiation of GSC lineage. We found that Tut binds preferentially a long isoform of <i>mei-P26</i> 3′UTR, and is essential for the translational repression of <i>mei-P26</i> reporter. Bam and Bgcn are both RBPs that have also been shown to repress <i>mei-P26</i> expression. Our genetic analyses indicate that <i>tut</i>, <i>bam</i>, or <i>bgcn</i> is required to repress <i>mei-P26</i> and to promote the differentiation of GSCs. Biochemically, we demonstrate that Tut, Bam, and Bgcn can form a physical complex in which Bam holds Tut on its N-terminus and Bgcn on its C-terminus. Our <i>in vivo</i> and <i>in vitro</i> evidence illustrate that Tut acts with Bam, Bgcn to accurately coordinate proliferation and differentiation in <i>Drosophila</i> germline stem cell lineage.</p>