Phosphatidylinositol 4-kinases: hostages harnessed to build panviral replication platforms

Abstract

Several RNA viruses have recently been shown to hijack members of the host phosphatidylinositol (PtdIns) 4-kinase (PI4K) family of enzymes. They use PI4K to generate membranes enriched in phosphatidylinositide 4-phosphate (PtdIns4P or PI4P) lipids, which can be used as replication platforms. Viral replication machinery is assembled on these platforms as a supramolecular complex and PtdIns4P lipids regulate viral RNA synthesis. This article highlights these recent studies on the regulation of viral RNA synthesis by PtdIns4P lipids. It explores the potential mechanisms by which PtdIns4P lipids can contribute to viral replication and discusses the therapeutic potential of developing antiviral molecules that target host PI4Ks as a form of panviral therapy.

Figure 1

PtdIns4P lipids are produced by phosphorylation of the precursor lipid PtdIns. Yeast and mammalian cells have several different PI4Ks (Table 1), which all produce the same enzymatic product: PtdIns4P. PtdIns4P can be converted back to PtdIns by Sac1 phosphatase.

Figure I

Domain organization of PI4Ks. (a) Mammalian PI4KIIIα and its yeast homolog, Stt4p. LKU, lipid kinase unique domain; PH, pleckstrin homology domain. (b) Mammalian PI4KIIIβ and its yeast homolog, Pik1p. hom2 refers to a domain that shows similarity to the LKU and is the site of binding Rab11. (c) Mammalian PI4KIIα and β and its yeast homolog, Lsb6p. The scale at the top of the figure refers to amino acid numbers.

Figure 2

The host cell secretory pathway is remodeled to generate PtdIns4P lipid enriched replication organelles. (a–c, 0 h) In uninfected cells the Golgi/TGN maintains a steady-state small pool of PI4KIIIβ and PtdIns4P lipids. (a–c, 2 h) 2 h post-infection with CVB3; the Golgi apparatus begins being resorbed into the ER, while new membranes begin emerging out of ER exit sites enriched in PI4KIIIβ and PtdIns4P lipids. (a–c, 4 h) By peak replication times, the Golgi is completely disassembled, secretory trafficking is blocked and new replication organelles (white arrows) highly enriched in PI4KIIIβ and PtdIns4P lipids are generated near ER exit sites. (d) These replication organelles serve as a platform for viral replication: viral replication proteins assemble on the cytosolic PtdIns4P rich leaflet of these membranes and synthesize viral RNA. The top panels are a sequence of confocal time-lapse images of a human cell infected with CVB3. In (a), the cell expresses FAPP1-PH-GFP, a live-cell PtdIns4P lipid reporter. In (b), native PI4KIIIβ is immunostained.

Figure 3

Biogenesis of PtdIns4P lipid replication platforms in enterovirus, kobuvirus and hepacivirus infections. Enterovirus replication protein 3A, kobuvirus replication protein 3A and hepacivirus (HCV) replication protein NS5A all hijack host PI4K family members. (a–c) Enterovirus and kobuvirus begin building replication platforms on the host Golgi/TGN, whereas HCV initiates replication platforms on the host endoplasmic reticulum (ER) (i). (a,b) Both enterovirus and kobuvirus 3A proteins recruit PI4KIIIβ through an intermediary host Golgi protein ACBD3 (ii). (a) In enterovirus infections, a rising level of viral 3A proteins (due to viral replication) results in enhanced PI4KIIIβ recruitment to Golgi/TGN membranes and concomitant loss of the coat proteins COPI and clathrin (ii). This leads to the eventual disassembly of the Golgi apparatus and relocation of replication machinery to newly formed organelles that have emerged from ER exit sites (iii). (b) In kobuvirus infections, although PI4KIIIβ recruitment is enhanced and the Golgi is disassembled, the fates of coat and GBF1–Arf1 proteins are yet unknown (ii). (c) In HCV-infected cells PI4KIIIα and/or PI4KIIIβ enzymes are required for replication and the HCV NS5A protein selectively enhances the recruitment of PI4KIIIα to the ER membrane (ii). (a–c) In entero-, kobu- and hepacivirus infections, recruited PI4K enzymes catalyze the formation of high levels of PtdIns4P lipids within membrane bilayers. High levels of PtdIns4P lipids can modify the structure and organization of the membrane platform (iii). This can regulate viral replication through electrostatic interactions and/or by recruiting other host PtdIns4P-binding proteins, which in turn may further differentiate the membrane into specialized microdomains to which viral proteins can partition into, concentrate and assemble into a supramolecular complex for RNA synthesis. Furthermore, PtdIns4P lipids themselves potentially help recruit viral proteins through direct interaction (e.g., enteroviral RdRp).