Selective suppression of mRNA translation by DAP5 controls cell fate decisions in human Embryonic Stem Cells

DAP5 is a member of the eIF4G protein family of translation initiation factors previously reported to promote cap-independent translation mediated by internal ribosome entry site elements. DAP5 knock-down in human embryonic stem cells (hESCs) prevents the induction of differentiation towards different cell lineages, while maintaining sustained expression of pluripotent genes. Polysomal profiling identified mRNAs with reduced translation efficiency (TE) in DAP5 knocked-down hESCs and mRNAs with elevated TE. To get a wider view of DAP5 interacting mRNAs we performed immunoprecipitation of DAP5 from UV-cross-linked hESCs followed by deep RNA sequencing. We identified a group of mRNAs to which DAP5 directly binds, and a group of DAP5 depleted mRNAs, probably reflecting indirect binding through other DAP5 interacting proteins. These two groups differed in their protein functional annotation and in their mRNAs physical attributes. Interestingly, we found high degree of overlap between the DAP5 bound mRNAs and the mRNAs with elevated TE in DAP5 knocked-down hESCs, indicating that direct binding of DAP5 to these mRNAs suppresses their translation. Conversely, the group of mRNAs with reduced TE overlapped with DAP5 depleted mRNAs. DAP5 function as a suppressor of translation was further supported by immunostaining data showing its localization to stress granules (SG). In addition, by data mining we found high overlap between DAP5 bound mRNAs and mRNA present in SG. We suggest that the translation suppression by direct RNA binding reflects a novel role for DAP5 in retaining a specific group of transcripts, such as mRNAs coding for pluripotency proteins, in untranslatable state in SG to allow differentiation.