PADI4 is a peptidyl-
arginine deiminase (PADI) involved in the conversion of
arginine to
citrulline. PADI4 is present in macrophages, monocytes, granulocytes, and several
cancer cells. It is the only PADI family member observed within both the nucleus and the cytoplasm. PADI4 has a predicted nuclear localization sequence (NLS) comprising residues Pro56 to Ser83, to allow for nuclear translocation. Recent predictors also suggest that the region Arg495 to Ile526 is a possible NLS. To understand how PADI4 is involved in
cancer, we studied the ability of intact PADI4 to bind
importin α3 (Impα3), a nuclear transport factor that plays
tumor-promoting roles in several
cancers, and its truncated species (ΔImpα3) without the
importin-binding domain (IBB), by using fluorescence, circular dichroism (CD), and isothermal titration calorimetry (ITC). Furthermore, the binding of two
peptides, encompassing the first and the second NLS regions, was also studied using the same methods and molecular docking simulations. PADI4 interacted with both
importin species, with affinity constants of ~1-5 µM. The isolated
peptides also interacted with both
importins. The molecular simulations predict that the anchoring of both
peptides takes place in the major binding site of Impα3 for the NLS of cargo
proteins. These findings suggest that both NLS regions were essentially responsible for the binding of PADI4 to the two
importin species. Our data are discussed within the framework of a cell mechanism of nuclear transport that is crucial in
cancer.