The O-Hyp glycosylation code in tobacco and Arabidopsis and a proposed role of Hyp-glycans in secretion

Abstract

Most aspects of plant growth involve cell surface hydroxyproline (Hyp)-rich glycoproteins (HRGPs) whose properties depend on arabinogalactan polysaccharides and arabinosides that define the molecular surface. Potential glycosylation sites are defined by an O-Hyp glycosylation code: contiguous Hyp directs arabinosylation. Clustered non-contiguous Hyp directs arabinogalactosylation. Elucidation of this code involved a single species, tobacco (Nicotiana tabacum) BY-2 cells. However, recent work suggests species variation, perhaps tissue specific Hyp glycosylation. Thus, the extent to which the Hyp glycosylation code is 'global' needs testing. We compared the ability of distantly related Arabidopsis cell cultures to process putative HRGP glycosylation motifs encoded by synthetic genes. The genes included: repetitive Ser-Pro, Ser-Pro2, Ser-Pro4 and an analog of the tomato arabinogalactan-protein, LeAGP-1DeltaGPI. All were expressed as enhanced green fluorescent protein (EGFP) fusion glycoproteins, designated: AtSO-EGFP (O=Hyp), AtSO2-EGFP, AtSO4-EGFP and AtEGFP-LeAGP-1DeltaGPI, respectively. The Arabidopsis glycosylation patterns were essentially similar to those observed in Nicotiana: non-contiguous Hyp residues in AtSO-EGFP were glycosylated exclusively with arabinogalactan polysaccharides while contiguous Hyp in AtSO2-EGFP and AtSO4-EGFP was exclusively arabinosylated. Mixed contiguous and non-contiguous Hyp residues in AtEGFP-LeAGP-1DeltaGPI were also arabinosylated and arabinogalactosylated consistent with the code. However, slightly more arabinogalactosylated Hyp and less non-glycosylated Hyp in AtEGFP-LeAGP-1DeltaGPI than tobacco NtEGFP-LeAGP-1DeltaGPI suggested Arabidopsis prolyl hydroxylases have a slightly broader specificity. Arabidopsis Hyp-arabinogalactans differed from tobacco in decreased glucuronic acid content and lack of rhamnose. Yields of the EGFP fusion glycoproteins were dramatically higher than targeted EGFP lacking Hyp-glycomodules. This validates earlier suggestions that the glycosylation of proteins facilitates their secretion.