Answer: (C) Spike protein

The spike protein on the coronavirus surface specifically enables host cell infection by binding to ACE2 receptors and facilitating membrane fusion.

Option Breakdown

Receptor Protein (A)

Receptor proteins like ACE2 exist on host cells, serving as targets for viral attachment.​
They receive the virus but are not viral components required for infection.​

PDE (B)

PDE refers to phosphodiesterases, enzymes that hydrolyze cyclic nucleotides like cAMP or cGMP.​
No established role exists for PDE in coronavirus infection mechanisms.​

Spike Protein (C)

The spike (S) glycoprotein mediates coronavirus entry by binding host receptors via its S1 subunit and fusing membranes via S2.
This trimeric protein determines host range, infectivity, and serves as the primary vaccine target.

Polymerase Basic 2 (D)

PB2 forms part of the influenza virus RNA polymerase complex, handling cap-snatching for replication.​
Coronaviruses use their own RNA-dependent RNA polymerase (RdRp/nsp12), not PB2, for genome replication.​

Coronavirus spike protein host infection begins when S protein binds ACE2 receptors on human cells, enabling viral entry.

Spike Protein Function

The spike protein’s S1 subunit recognizes ACE2 while S2 drives fusion after protease cleavage (furin/TMPRSS2).
This two-step process—attachment then fusion—makes spike essential for infectivity across SARS-CoV-2 variants.

Why Not Other Options?

Receptor proteins reside on host cells, not virus particles.​
PDE regulates signaling pathways unrelated to viral entry; PB2 belongs to influenza, not coronaviruses.​

Research Applications

Spike mutations alter transmission and vaccine efficacy, as seen in variants like Omicron.
Structural studies guide therapeutics blocking spike-ACE2 interaction.​

This confirms C as the definitive answer for coronavirus host infection.