Sugar-mediated lattice contacts in crystals of a plant glycoprotein

Pokeweed antiviral protein, PAP-S_aci, isolated from seeds of the Chinese pokeweed plant, Phytolacca acinosa, belongs to the family of type-1 ribosome-inactivating proteins (RIPs). Type-1 RIPs are ∼30-kDa N-glycosidases that inactivate eukaryotic and prokaryotic ribosomes via a site-specific depurination of ribosomal RNA (rRNA). Here we describe the preliminary X-ray structure determination at 1.7 Å resolution of one PAP isoenzyme from seeds, PAP-S1_aci, after crystallisation from a heterogeneous mixture of two isoenzymes. PAP-S1_aci possesses a rare type of glycosylation, specifically, N-linked N-acetyl-D-glucosamine monosaccharide (GlcNAc) substitutions at canonical Asn-Xaa-Ser/Thr sequons. One GlcNAc residue was found to play a critical role in crystal lattice formation, forming a packing interface across a crystallographic two-fold with the identical sequon of an adjacent monomer. This observation suggests that deglycosylation protocols for the crystallisation of glycoproteins should be designed to allow for exploitation of the crystal packing potential of the innermost core sugar residue (N-linked GlcNAc or O-linked GalNAc).