Peptidyl arginine deiminase 6 (PADI6) is a maternal-effect gene essential for early embryo development and female fertility. PADI6 is the least well characterised of the PADIs - a family of enzymes that catalyse the post-translational conversion of peptidyl arginine to citrulline. Current literature suggests that the primary function of PADI6 in embryo development is structural, not catalytic, and PADI6 functions as a key structural component of the cytoplasmic lattices (CPLs). The CPLs are abundant structures, exclusive to the oocyte and early embryo, recently proposed to be a storage site for maternal proteins. Despite its clear biological and clinical significance, the exact function of PADI6 remains elusive. Crucially, there is currently no reported catalytic activity of PADI6.

In our work, we aimed to expand our understanding of the function of PADI6 in early embryo development, in particular, the question of whether PADI6 has a catalytic function. Using a pair of mouse models, we collected scRNA-seq data from oocytes and early embryos. Our data reveals that embryonic genome activation EGA is defective in embryos from Padi6 knock-out mothers, and also appears misregulated in embryos from mothers in which a mutation has been introduced into the PADI6 active site, despite their CPLs appearing intact. Together this suggests that there may be an additional non-essential function of PADI6 during EGA, independent of CPL formation. To further characterise these observations, we have established a single embryo proteomic workflow capable of consistently identifying upwards of 6000 proteins from single oocytes or embryos. Using this single embryo proteomic workflow, we have characterised the proteomes of GV oocytes, MII oocytes, zygotes and 2-cell embryos from each of our lines. These data further point towards an additional function of PADI6 in early embryo development, independent of the CPLs, which may involve catalysis.