PDCD2 functions as an evolutionarily conserved chaperone dedicated for the 40S ribosomal protein uS5

PDCD2 is an evolutionarily conserved protein that is essential for cell proliferation and embryonic development. To date, however, the function of PDCD2 underlying its fundamental cellular role has remained elusive. Here we used quantitative proteomics approaches to define the protein-protein interaction network of human PDCD2. Our data revealed that PDCD2 specifically interacts with the 40S ribosomal protein uS5 (RPS2) and that the PDCD2-uS5 complex is assembled co-translationally. Loss of PDCD2 expression leas to defects in the synthesis of small ribosomal subunit that phenocopies a uS5 deficiency. Notably, we showed that PDCD2 is important for the accumulation of soluble uS5 proteins as well as its incorporation into 40S ribosomal subunit. Our findings support that the essential molecular function of PDCD2 is to act as a dedicated chaperone that recognizes the ribosomal protein uS5 co-translationally in the cytoplasm and accompanies uS5 to ribosome assembly sites in the nucleolus. As most dedicated ribosomal protein chaperones have been identified in yeast, our study indicates that similar mechanisms exist in human cells to assist ribosomal proteins coordinate their folding, nuclear import, and assembly in pre-ribosomal particles. Overall design: Libraries for control (siNT) and PDCD2-depleted HeLa cells were prepared using total RNA from two independent knockdown experiments and analyzed using the Illumina HiSeq technology.