Hepatitis C Virus NS5A Protein–A Master Regulator? Gyongyi Szabo  Gastroenterology  Volume 130, Issue 3, Pages 995-999 (March 2006) DOI: 10.1053/j.gastro.2006.01.072 Copyright © 2006 American Gastroenterological Association Terms and Conditions

Figure 1 The schematic diagram of the hepatitis C virus genome and non-structural (NS) 5A protein. The Hepatitis C virus has a plus-strand, 9.6 kb RNA genome that carries a single long open reading frame (ORF) with 5′ and 3′ flanked ends required for RNA translation and replication. The viral genes are expressed as a polyprotein, 3,000 amino acids in length, which is cleaved co- and post-translationally by host cell signal peptidases and two viral proteinases. At least 10 different cleavage products have been identified, which are ordered within the polyprotein: NH2-core-E1-E2-p7-NS2-NS3-NS4A-NS4B-NS5A-NS5B-COOH. The NS5A protein has a variety of functions, including participation in polyprotein clevage and interaction with host proteins and thus modulation of host homeostasis. NS5A has 447 aa and is divided into 2 low complexity sequences. The domain organization model for HCV NS5A includes domain I (amino acids 1–213), domain II (amino acids 250–342), and domain III (amino acids 355–447). The N-terminal 30 aa forms a highly conserved amphipathic α-helix necessary for association of NS5A with the endoplasmic reticulum membrane (membrane anchor). The NS5A protein is highly phosphorylated and residues 2200–2250 and 2350–2419, also called hyperphosphorylation cluster, are required for basal phosphorylation. The NS5A protein also contains: (1) An unconventional, cysteine-rich zinc binding motif within the N-terminal domain (Zn binding site, aa 36–100) that is vital for HCV replication. (2) An interferon sensitivity-determining region (ISDR, amino acids 237–276), associated with IFN resistance and lack of response to treatment in genotype 1a-infected patients. The 40-amino-acid ISDR and 26 amino acids downstream of the ISDR on NS5A constitute the IFN-induced, double-stranded RNA-activated protein kinase (PKR) binding domain. (3) A polyproline cluster, containing 2 classes of proline-rich motifs: Class I (consensus sequence KxxPxxP) and Class II (consensus PxxPxR), distinguished by the orientation adopted by the motif when bound to an Src homology 3 (SH3) domain of a number of cellular signaling proteins including Grb2, Amphiphysin II and Src-family tyrosine kinases. (4) A nuclear localization signal (NLS, aa 354 to 362) that does not direct NS5A to the nucleus by itself, but is nonetheless functional in directing nuclear translocation when placed at the amino terminus of a reporter gene. Bin-Amphiphysin-Rvs (BAR) domain, bridging integrator protein (Bin1) 1-binding site, and methyl-CpG binding domain (MBD) are shown in the diagram. Gastroenterology 2006 130, 995-999DOI: (10.1053/j.gastro.2006.01.072) Copyright © 2006 American Gastroenterological Association Terms and Conditions