Virus Intracellular Movement LECTURE 11: Viro100: Virology 3 Credit hours NUST Centre of Virology & Immunology Waqas Nasir Chaudhry

Intracellular transport After successful attachment and entry The virus, or at least its genome, may have to be delivered to a particular location i.e. Nucleus Destination is reached using one of the transport systems of the cell Microtubules

Influenza Virus entry and replication

Influenza viruses bind through hemagglutinin onto sialic acid sugars on the surfaces of epithelial cells Typically in the nose, throat, and lungs of mammals, and intestines of birds Cell imports the virus by endocytosis The endosome interact with acidic endosomes The endosome moves on the microtubule to reach to nucleus

The acidic conditions in the endosome cause two events to happen; First, part of the hemagglutinin protein fuses the viral envelope with the vacuole's membrane Then the M2 ion channel allows protons to move through the viral envelope and acidify the core of the virus Which causes the core to dissemble and release the viral RNA and core proteins The viral RNA (vRNA) molecules, accessory proteins and RNA dependent RNA polymerase are then released into the cytoplasm

The M2 ion channel is blocked by amantadine drugs, preventing infection

RNA Viruses Most RNA viruses of eukaryotes replicate in the cytoplasm; the majority encode all the enzymes for replication of their genomes and they have no requirement for the enzymes of the nucleus. Influenza viruses are exceptions as they require the cell splicing machinery, so their genomes must be delivered into the nucleus.

Retroviruses Retroviruses are RNA viruses that replicate their genomes in the nucleus. They copy their genomes to DNA in the cytoplasm, then move their DNA in nucleus. Lentiviruses i.e. HIV Influenza virus require dividing cell for genome transport to nucleus (Mitosis) HIV can replicate in non dividing cells

DNA Virus Most DNA viruses replicate in the nucleus Some DNA viruses, such as iridoviruses and poxviruses, replicate in the cytoplasm of eukaryotic cells The virus genome must be transported to the nuclear envelope and then across it Example are adenoviruses and herpes viruses

Q; Why does DNA virus replicate in the nucleus, and RNA viruses replicate in the cytoplasm? Ans; The nucleus of eukaryotic cells is where DNA is replicated in a normal cell. A DNA virus replicating in this cell would then have to replicate in the nucleus since all the proteins for DNA replication and RNA transcription are there. RNA on the other hand is translated in the cytoplasm by ribosomes on the endoplasmic reticulum. Since RNA viruses don't need to transcribe DNA to RNA (which happens in the nucleus) they can just start the translation process in the cytoplasm.

Virus Movement inside the Cell

Microtubules Microtubules are components of the cytoskeleton, providing support for various components of the cell and acting as tracks for the transport of materials, such as certain organelles, to particular sites in the cell Microtubules are hollow cylinders, 25 nm in diameter, and are composed of the proteins α- and β-tubulin.

Network of Microtubule in the Cell

Proteins, known as motor proteins, move themselves and cargo along the microtubules. A number of viruses (including herpesviruses, adenoviruses, parvoviruses and retroviruses) exploit this transport system to take their nucleocapsid, or a structure derived from it, from the periphery of the cytoplasm to a location close to the nucleus. The virus structures are transported at speeds of 1– 4 µm per second

Nuclear Pore Complex Each nuclear pore is constructed from a complex of more than 50 protein species The protein molecules are assembled into discrete structures, including a ‘basket’ protruding into the nucleus and eight filaments that protrude into the cytoplasm