No Envelope: Penetration and Lysis | AI Learning Guide

Without an envelope there is nothing to fuse, so the capsid must breach the membrane itself. In penetration, a lytic peptide inserts into the endosomal membrane and creates a pore, and the capsid or a partially uncoated particle slips through. In lysis, the disruption is more extensive and the genome is released directly. The important point is that the lytic peptide is buried in the intact capsid and only becomes exposed after the right trigger, usually low pH in the endosome. That is what keeps the virus from damaging membranes it has not yet reached. It also means the capsid is not a passive box: its stability and trigger responsiveness are tuned to the route, and uncoating at the wrong time or place fails to deliver the genome.

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Penetration versus lysis

Membrane penetration

Lytic peptide inserts into the endosomal membrane
Creates a pore or local disruption
Capsid or subviral particle passes into the cytosol
Membrane remains largely intact

Membrane lysis

Capsid disrupts the membrane more extensively
Genome released directly into the cytosol
Membrane integrity lost
Often used at the plasma membrane or late endosome

The trigger protects the virus from itself

The lytic peptide is buried in the intact capsid and exposed only after the right trigger — low pH or receptor binding. This ensures the virus damages only the membrane it has already reached, not the membranes it encounters earlier.