Attachment and Entry – Key Vocabulary (Lecture 3)

Vocabulary flashcards summarizing essential terms related to viral attachment, entry, intracellular trafficking, and nuclear import, based on Lecture 3 notes.

Adhesin

Viral surface protein that specifically binds a cellular receptor to initiate attachment.

Cellular Receptor

Host cell molecule (often a protein or glycan) recognized by a viral adhesin for entry.

Co-receptor

Additional host molecule required, alongside the primary receptor, to permit viral entry (e.g., CXCR4 or CCR5 for HIV-1).

Tropism

The range of cell types, tissues, or species a virus can infect, determined largely by receptor expression.

Glycosaminoglycans (GAGs)

Negatively charged polysaccharides (e.g., heparan sulfate) in the ECM that mediate cell-cell attachment and can provide low-affinity virus binding sites.

Proteoglycans

Proteins covalently linked to GAGs; form hydrated gels between cells and participate in virus attachment.

Extracellular Matrix (ECM)

Network of proteins and polysaccharides (collagen, laminin, fibronectin, elastin) supporting tissue structure and influencing viral spread.

Collagen

Rope-like ECM protein providing tensile strength and attachment points (e.g., for Nidogen) in the basal lamina.

Laminin

ECM glycoprotein that binds collagen, integrins, and GAGs; helps maintain basal lamina integrity.

Fibronectin

ECM protein that bridges collagen, integrins, and GAGs; supports cell adhesion and can be exploited by viruses.

Integrin

Transmembrane heterodimer (α/β) mediating cell-ECM adhesion; serves as receptor or co-receptor for several viruses (e.g., αVβ3 for FMDV).

ICAM-1

Immunoglobulin superfamily adhesion molecule; primary receptor for most rhinoviruses and stabilizer of immune cell interactions.

Selectins

Carbohydrate-binding adhesion molecules (Ig superfamily) that assist leukocyte trafficking; potential virus attachment factors.

CAR

Coxsackie-Adenovirus Receptor; Ig-like protein on basolateral surfaces bound by many adenoviruses and coxsackieviruses.

PVR

Poliovirus Receptor (CD155); Ig-like protein used by polio and some α-herpesviruses as a high-affinity entry receptor.

Sialic Acid

Terminal monosaccharide on glycans; recognized by influenza HA (α2,3 linkage for avian strains, α2,6 for human strains).

Hemagglutinin (HA)

Influenza envelope glycoprotein (HA1/HA2) that mediates receptor binding and, after acidification, membrane fusion.

Fusion Peptide

Hydrophobic segment within a viral fusion protein (e.g., F1 of paramyxoviruses, HA2 of influenza) that inserts into host membranes during fusion.

Receptor-Mediated Endocytosis

Cellular uptake mechanism involving clathrin-coated pits that viruses hijack for entry.

Clathrin-Coated Pit

Specialized plasma-membrane invagination where receptor-virus complexes cluster before endocytosis.

Dynamin

GTPase that pinches off clathrin-coated pits to form endocytic vesicles (endosomes).

Endosome

Membrane-bound vesicle formed by endocytosis; acidifies and often triggers viral uncoating or fusion.

Lysosome

Acidic vesicle containing proteases; fusion with endosome can uncoat certain viruses (e.g., reoviruses).

Fusion Pore

Transient opening created by viral fusion proteins allowing contents of two membranes to mix.

Acidification

Proton accumulation that drops endosomal pH, activating conformational changes in many viral proteins (e.g., influenza HA).

Cytoplasmic Streaming

Microtubule-based active transport of vesicles or capsids using motor proteins kinesin and dynein.

Microtubules

Polar cytoskeletal filaments (- to + ends) serving as tracks for intracellular viral transport.

Kinesin

ATP-driven motor that moves cargo toward the microtubule + end (usually cell periphery).

Dynein

ATP-driven motor that moves cargo toward the microtubule ‑ end (usually nucleus).

Uncoating

Process by which a viral genome is released from its protective capsid or envelope inside the host cell.

Ribonucleoprotein (RNP)

Complex of viral RNA with associated proteins (e.g., NP of influenza) that must reach replication sites.

Capsid

Protein shell protecting the viral genome; undergoes conformational changes or disassembly during entry.

Nucleocapsid

Capsid bound directly to viral nucleic acid; released into cytosol or delivered to nucleus after membrane fusion or pore formation.

Transcytosis

Vectorial transport of viruses from the apical to basolateral surface (or vice versa) across polarized cells.

Apical Surface

Epithelial cell membrane facing the external environment; common site of initial viral attachment.

Basolateral Surface

Epithelial cell side contacting underlying tissues; infection here can promote systemic spread.

Tight Junction

Intercellular seal that maintains cell polarity and restricts viral access between apical and basolateral domains.

Simple NLS

Short, contiguous stretch of basic residues that directs proteins (or viral genomes) to the nucleus.

Bipartite NLS

Two clusters of basic residues separated by a spacer; also mediates nuclear import.

Importin-α

Adaptor that binds the NLS of cargo proteins and recruits importin-β for nuclear transport.

Importin-β

Transport receptor that interacts with importin-α–cargo complexes and the nuclear pore complex.

Ran-GTP

GTP-bound form of Ran that drives directionality of nuclear import/export by regulating cargo release.

RCC1

Chromatin-bound guanine nucleotide exchange factor that converts Ran-GDP to Ran-GTP inside the nucleus.

RanGAP

Cytoplasmic GTPase-activating protein that hydrolyzes Ran-GTP, resetting the import/export cycle.

Integrase (Int)

Retroviral enzyme that remains bound to newly synthesized dsDNA; may carry an NLS for nuclear entry in non-dividing cells.

Cyclophilin A

Host chaperone packaged by HIV-1; facilitates capsid (CA) disassembly after membrane fusion.

F Protein

Paramyxovirus fusion glycoprotein (F1/F2) that mediates pH-independent membrane fusion at the plasma membrane.

LDL-like Receptor

Cell surface protein involved in cholesterol uptake; serves as an alternative receptor for some rhinoviruses.

MHC Class I/II

Antigen-presenting molecules; Class I acts as an adenovirus co-receptor, Class II interacts with CD4 on T cells targeted by HIV.

RGD Motif

Arg-Gly-Asp sequence recognized by several integrins (e.g., αVβ3, αVβ5) and exploited by adenoviruses for co-receptor binding.

αVβ3 Integrin

Cellular integrin acting as a receptor for some foot-and-mouth disease virus strains and adenovirus co-receptor.

αVβ5 Integrin

Integrin class frequently serving as an adenovirus co-receptor during cell entry.

V1–V5 Loops

Variable regions of HIV-1 gp120 (SU) that form the CD4 binding site and influence co-receptor specificity.

HA1

Globular subunit of influenza HA responsible for sialic-acid binding.

HA2

Stalk subunit of influenza HA containing the fusion peptide exposed at low pH.

Reverse Transcriptase (RT)

Retroviral enzyme that converts viral RNA into dsDNA prior to nuclear import and integration.