RpS12-mediated induction of the Xrp1 ^short isoform links ribosomal protein mutations to cell competition

Cell competition, a universal yet enigmatic phenomenon, eliminates less-fit cells via interactions with their neighbors. It was originally described in Drosophila mosaics, where heterozygous ribosomal protein ( Rp ^+/- ) mutant cells are eliminated by wild-type neighbors. The transcription factor Xrp1 mediates most of the Rp ^+/- -associated phenotypes, including reduced competitiveness and translation. Although RpS12 is required for Xrp1 induction in Rp ^+/- cells, the mechanism remained unresolved. We demonstrate that RpS12, via alternative splicing, induces the Xrp1 short (Xrp1 ^short ) isoform expression in Rp ^+/- cells, which is both necessary and sufficient for their elimination. Strikingly, RpS12 overexpression in wild-type cells is sufficient to induce Xrp1 ^short expression and confer a "loser" phenotype. While Xrp1 ^long isoform is not required in Rp ^+/- cells, expression of either Xrp1 isoform is sufficient to promote the loser status in wild-type cells. We further identify Syncrip, an RNA-binding protein reduced in Rp ^+/- cells, as a critical Xrp1 suppressor; its depletion in wild-type cells activates Xrp1-dependent competition. Our findings establish RpS12's specialized function in Xrp1 ^short promotion, not proteotoxic stress, as the primary driver in Rp ^+/- cells, providing new perspectives that challenge and refine prevailing models. Our work contributes in long-standing questions about ribosomal protein-linked fitness surveillance and provides insights into ribosomopathy pathologies.