Which viral protein of influenza virus is responsible for binding to sialic acid receptors on host respiratory epithelial cells?

Explanation

## Influenza Virus Surface Proteins and Receptor Binding **Key Point:** Hemagglutinin (HA) is the major surface glycoprotein of influenza virus that mediates initial attachment to host cells by binding to sialic acid (N-acetylneuraminic acid) receptors on respiratory epithelial cells. ### Structural and Functional Roles | Protein | Function | Location | |---------|----------|----------| | **Hemagglutinin (HA)** | Binds sialic acid receptors; mediates cell entry | Surface spike | | **Neuraminidase (NA)** | Cleaves sialic acid; enables virion release | Surface spike | | **Matrix protein (M1)** | Structural support; virion assembly | Inner layer | | **NS1** | Antagonizes interferon response | Non-structural | **High-Yield:** HA is the primary target of neutralizing antibodies and is the basis for vaccine strain selection. The HA protein exists as a homotrimer on the virion surface, with each monomer containing two functional domains: a globular head (receptor-binding domain) and a fibrous stalk (membrane-proximal domain). ### Mechanism of Cell Entry 1. HA binds to sialic acid receptors (α-2,6 linked in humans; α-2,3 linked in birds) 2. Receptor binding triggers conformational change 3. Membrane fusion occurs via the HA2 subunit (fusion peptide) 4. Viral ribonucleoprotein complex enters cytoplasm **Clinical Pearl:** Antigenic drift in HA and NA genes leads to seasonal influenza epidemics, while antigenic shift (reassortment of gene segments) can generate pandemic strains. **Mnemonic:** **HA-NA rule** — **HA** = **H**ost **A**ttachment (binding); **NA** = **N**euraminidase **A**ctivity (release).