Surface charge distribution of Hv PCNA compared with Af PCNA. A. Electrostatic surfaces of Hv (left) and Af PCNA (right [PDB:1RWZ]) demonstrate that the acidic nature of PCNAs is more pronounced in Hv PCNA and that the halophilic protein lacks the positive electrostatic charge characteristic of the inner channel. The electrostatic potential was calculated using the APBS package . The accessible surface area is coloured according to the calculated electrostatic potential from -10 kBT/e (red) to +10 kBT/e (blue). B. Penetration of basic residues into the central channel of Hv PCNA (top) and Af PCNA (bottom). The structures are depicted with a backbone trace with basic residues located on the α-helices lining the central pore depicted in stick representation. In Hv PCNA only Lys143 and Lys205 project into the channel in the manner seen in classical PCNAs. Arg12, Arg72 and Arg140 are involved in substantial interactions with protein atoms and Lys201 is involved in charge neutralisation at the sodium cluster site. In contrast the majority of the basic residues lining the Af PCNA pore project into the channel.