Possesses antiviral potency. Inhibits viral infection of plants (tobacco mosaic virus) (PubMed:10403789). Inhibits protein synthesis in both prokaryotes and eukaryotes (PubMed:8034016, PubMed:10403789). Endohydrolysis of the N-glycosidic bond at one specific adenosine on the 28S rRNA. PAP-II is expressed in early summer leaves (at protein level) (PubMed:10403789). PAP-III is expressed in late summer leaves (at protein level) (PubMed:10403789, PubMed:12754107). Expressed progressively with the aging of the plant. 2 forms exist, PAP-II and PAP-III, that differ in their seasonal expression and chromatographic profiles. Both depurinate genomic RNA of immunodeficiency virus type-I (HIV-I) and bacteriophage (MS 2) RNA. PAP-II and PAP-III inhibit the replication of HIV-1 in human peripheral blood mononuclear cells with IC(50) values of 26 nM and 17 nM, respectively. Belongs to the ribosome-inactivating protein family. Type 1 RIP subfamily.