Unveiling nuclear localization signals in human arginine deiminase proteins

Arginine iminohydrolases are a family of enzymes involved in the conversion of arginine to citrulline. There are five isoforms in humans (PADI1, 2, 3, 4, and 6). Some of them are observed experimentally in the cytoplasm and in the nucleus of the cell; for moving to the latter location, they must pass through the cell nuclear membrane by using the translocation machinery, mainly formed by the proteins named importins. We have previously described and characterized the isolated PADI4 nuclear localization sequences (NLSs) and their binding to importin α3 (Impα3). By using theoretical predictors, here we foretold the existence of several NLSs in isoforms PADI1, PADI2, and PADI3. These predicted polypeptide regions were chemically synthesized, and the soluble ones were conformationally characterized in isolation. We studied their ability to bind Impα3 and its truncated species (ΔImpα3) without the importin binding domain, by using several biophysical techniques and molecular simulations. The isolated peptides were disordered and monomeric in solution. Moreover, all of them were capable of binding to both importin species with affinities in the low micromolar range, and targeting the canonical NLS binding site for cargo proteins. These findings suggest that the predicted NLS regions could be the sites for the binding of the corresponding intact PADI protein to importin, and therefore, any of the PADI enzymes could be translocated into the nucleus.