Understanding Viral Entry: How the Gn and Gc Spikes Bind to Hosts

Understanding Viral Entry: How the Gn and Gc Spikes Bind to Hosts

Orthohantaviruses are a group of viruses carried by rodents that can cause serious human illness, including hantavirus pulmonary syndrome (HPS) and hemorrhagic fever with renal syndrome (HFRS). Central to their ability to infect human cells are two envelope glycoproteins—Gn and Gc—that form spike complexes on the virus surface. In this overview, we explore how these spikes recognize and enter host cells, drawing on studies published in peer-reviewed journals and guidance from public health agencies.

Orthohantavirus Envelope Glycoproteins: Gn and Gc at a Glance

• Gn (glycoprotein N)
  • Sits at the tip of the spike, involved in initial receptor engagement.
  • Contains immunodominant regions recognized by the immune system.
• Gc (glycoprotein C)
  • Lies closer to the viral membrane, responsible for membrane fusion.
  • Undergoes large conformational changes at low pH to drive entry.
• Spike Complex Formation
  • Gn and Gc assemble into tetrameric spikes visible under cryo-EM.
  • Each spike spans the viral membrane and extends outward ~10–12 nm.

These envelope glycoproteins are collectively referred to as "Orthohantavirus envelope glycoproteins" in the virology literature. Together, they orchestrate the critical steps of attachment, internalization, and fusion that allow the viral genome to enter host cells.

Step 1: Attachment to Host Cell Receptors

Entry begins when the Gn spike binds to specific receptors on the surface of human cells. Key receptor interactions include:

• β3 integrins (αvβ3 and αIIbβ3)
  – Identified on endothelial and immune cells.
  – Recognize an RGD (arginine-glycine-aspartic acid) motif on Gn.
• Protocadherin-1 (PCDH1)
  – A cadherin family protein found in lung endothelial cells.
  – Crucial for New World orthohantaviruses (e.g., Sin Nombre virus).
• Glycosaminoglycans (GAGs)
  – Heparan sulfate and other sugars can enhance binding in some strains.

By binding these receptors, the virus gains a foothold on the cell surface. This initial contact triggers clustering of the spike complexes and primes the virus for entry.

Step 2: Internalization via Endocytosis

After attachment, orthohantaviruses are brought into the cell through endocytosis—a process cells normally use to internalize nutrients and other particles. Key points:

• Clathrin-mediated endocytosis
  • Most common route for Old World hantaviruses (e.g., Hantaan virus).
  • Involves the protein clathrin forming a coated pit around the virus.
• Caveolin-mediated pathways
  • Used by some New World strains.
  • Caveolin proteins shape flask-like pits that pinch off into vesicles.
• Macropinocytosis
  • A "cell drinking" process that captures larger volumes of fluid and particles.
  • Sometimes contributes to hantavirus uptake in immune cells.

Inside the resulting endosomal vesicle, the virus is enclosed in a membrane-bound compartment separate from the cytosol.

Step 3: pH-Triggered Fusion of Viral and Endosomal Membranes

The endosomal environment is acidic (pH ~5.5–6.0). This low pH is the cue for Gc to refold and merge the viral envelope with the endosomal membrane:

• Gc conformational change
  • Acidic conditions trigger a "spring-loaded" shift in Gc's structure.
  • Gc exposes a fusion loop that inserts into the host membrane.
• Hemifusion and pore formation
  • Viral and endosomal membranes merge outer leaflets first (hemifusion).
  • A fusion pore opens, allowing the viral ribonucleoprotein to enter the cytosol.
• Release of the viral genome
  • Upon fusion pore expansion, the negative-sense RNA segments are released.
  • Viral replication and protein synthesis begin in the cytoplasm.

This pH-dependent fusion mechanism is a shared strategy among many enveloped viruses and is a target for antiviral drug development.

Why Gn and Gc Matter for Immunity and Vaccines

Since Gn and Gc spikes are exposed on the virus surface, they are the primary targets of the host immune response:

• Neutralizing antibodies
  • Mainly directed against Gn's receptor-binding regions and Gc's fusion loop.
  • Block spike conformational changes or receptor engagement.
• T-cell responses
  • Recognize viral peptides derived from Gn/Gc during infection.
  • Contribute to viral clearance and long-term immunity.
• Vaccine design
  • Many experimental vaccines present Gn and Gc in recombinant form.