Proteomic Profiling of Unannotated Microproteins in Human Placenta Reveals XRCC6P1 as a Potential Negative Regulator of Translation

Ribosome
profiling and mass spectrometry have revealed
thousands
of previously unannotated small and alternative open reading frames
(sm/alt-ORFs) that are translated into micro/alt-proteins in mammalian
cells. However, their prevalence across human tissues and biological
roles remains largely undefined. The placenta is an ideal model for
identifying unannotated microproteins and alt-proteins due to its
considerable protein diversity that is required to sustain fetal development
during pregnancy. Here, we profiled unannotated microproteins and
alt-proteins in human placental tissues from preeclampsia patients
or healthy individuals by proteomics, identified 52 unannotated microproteins
or alt-proteins, and demonstrated that five microproteins can be translated
from overexpression constructs in a heterologous cell line, although
several are unstable. We further demonstrated that one microprotein,
XRCC6P1, associates with translation initiation factor eIF3 and negatively
regulates translation when exogenously overexpressed. Thus, we revealed
a hidden sm/alt-ORF-encoded proteome in the human placenta, which
may advance the mechanism studies for placenta development as well
as placental disorders such as preeclampsia.