Interactions of the Disordered Domain II of Hepatitis C Virus NS5A with Cyclophilin A, NS5B, and Viral RNA Show Extensive Overlap.

Domain II of the nonstructural protein 5 (NS5A) of the hepatitis C virus (HCV) is involved in intermolecular interactions with the viral RNA genome, the RNA-dependent RNA polymerase NS5B, and the host factor cyclophilin A (CypA). However, domain II of NS5A (NS5A) is largely disordered, which makes it difficult to characterize the protein-protein or protein-nucleic acid interfaces. Here we utilized a mass spectrometry-based protein footprinting approach in attempts to characterize regions forming contacts between NS5A and its binding partners. In particular, we compared surface topologies of lysine and arginine residues in the context of free and bound NS5A. These experiments have led to the identification of an RNA binding motif (RSRKFPR) in an arginine-rich region of NS5A. Furthermore, we show that K308 is indispensable for both RNA and NS5B binding, whereas W316, further downstream, is essential for protein-protein interactions with CypA and NS5B. Most importantly, NS5A binding to NS5B involves a region associated with RNA binding within NS5B. This interaction down-regulated RNA synthesis by NS5B, suggesting that NS5A modulates the activity of NS5B and potentially regulates HCV replication.