ERBB3‐Binding Protein 1 (EBP1) Is a Novel Developmental Pluripotency‐Associated‐4 (DPPA4) Cofactor in Human Pluripotent Cells

Developmental Pluripotency‐Associated‐4 ( DPPA4 ) is one of the few core pluripotency genes lacking clearly defined molecular and cellular functions. Here, we used a proteomics screening approach of human embryonic stem cell (hESC) nuclear extract to determine DPPA4 molecular functions through identification of novel cofactors. Unexpectedly, the signaling molecule ERBB3‐binding protein 1 (EBP1) was the strongest candidate binding partner for DPPA4 in hESC. EBP1 is a growth factor signaling mediator present in two isoforms, p48 and p42. The two isoforms generally have opposing functions, however their roles in pluripotent cells have not been established. We found that DPPA4 preferentially binds p48 in pluripotent and NTERA‐2 cells, but this interaction is largely absent in non‐pluripotent cells and is reduced with differentiation. The DPPA4–EBP1 interaction is mediated at least in part in DPPA4 by the highly conserved SAF‐A/B, Acinus and PIAS (SAP) domain. Functionally, we found that DPPA4 transcriptional repressive function in reporter assays is significantly increased by specific p48 knockdown, an effect that was abolished with an interaction‐deficient DPPA4 ΔSAP mutant. Thus, DPPA4 and EBP1 may cooperate in transcriptional functions through their physical association in a pluripotent cell specific context. Our study identifies EBP1 as a novel pluripotency cofactor and provides insight into potential mechanisms used by DPPA4 in regulating pluripotency through its association with EBP1. S tem C ells 2018;36:671–682