Identification of the Pokeweed Antiviral Protein Interactome by Co-Immunoprecipitation-Mass Spectrometry

Ribosome-inactivating proteins (RIPs) are produced primarily by plants and are named for their enzymatic ability to depurinate ribosomal RNA. RIPs have been shown to have antiviral, antifungal, and antibacterial activity in vitro and when expressed transgenically. They are therefore of interest for their potential in human health, as both pathogenic agents and therapeutics, as well as in agriculture, to confer disease resistance in transgenic crops. However, little is known about the biological function of RIPs in their native context. Phytolacca americana, the American pokeweed, produces a RIP called pokeweed antiviral protein (PAP). The objective of this work is to investigate the role of PAP by mapping out the PAP-protein interactome; this will elucidate PAP’s function by implicating the processes in which it is involved. Co-immunoprecipitation coupled with mass spectrometry was used to identify PAP protein interactors in pokeweed. Results identified protein interactions with diverse cellular functions in both the extracellular matrix, where PAP is primarily localized, and the cytoplasm, where the ribosomal target resides. One interactor was identified as a probable extracellular cysteine protease (paCP1); since this protein class has known roles in plant defense, paCP1 was chosen for further validation of its interaction with PAP using reverse co-IP. Differential expression and in silico promoter analysis demonstrated PAP and paCP1 co-expression in response to jasmonic acid, supporting the role of this interaction in defense. This work represents the first protein interactome mapping for a RIP; identification of PAP interactors in plant cells contributes to understanding PAP function and will aid in characterizing the biological role of RIPs in general.