Seasonal variations in different forms of pokeweed antiviral protein, a potent inactivator of ribosomes.

Pokeweed antiviral proteins enzymatically inactivate the 60 S subunit of eucaryotic ribosomes in cell-free preparations. Three different species of the enzyme can be isolated from spring leaves, summer leaves, and seeds of pokeweed. Sequence analyses of the NH2-terminal residues show that pokeweed antiviral protein, isolated from spring leaves and seeds, are homologous and differ in 11 of the 28 residues compared. Ricin contains a polypeptide (ricin A chain) that has functional similarities to pokeweed antiviral protein, yet the sequences of the pokeweed proteins show little similarity with ricin A chain. Ricin B chain is responsible for helping ricin A chain across the plasma membrane; since pokeweed antiviral has no counterpart to ricin B chain, it is not nearly as cytotoxic as ricin. However, when pokeweed antiviral protein was covalently coupled to ricin B chain, a cytotoxic species was formed. Pokeweed antiviral protein fails to interact noncovalently with the ricin B chain to produce a cytotoxic species equivalent in function to ricin.