Abstract

Arabidopsis EARLI-type hybrid proline-rich proteins (HyPRPs) consist of a putative N-terminal secretion signal, a proline-rich domain (PRD), and a characteristic eight-cysteine-motif (8-CM). They have been implicated in biotic and abiotic stress responses. AZI1 is required for systemic acquired resistance and it has recently been identified as a target of the stress-induced mitogen-activated protein kinase MPK3. AZI1 gel migration properties strongly indicate AZI1 to undergo major post-translational modifications. These occur in a stress-independent manner and are unrelated to phosphorylation by MAPKs. As revealed by transient expression of AZI1 in Nicotiana benthamiana and Tropaeolum majus, the Arabidopsis protein is similarly modified in heterologous plant species. Proline-rich regions, resembling arabinogalactan proteins point to a possible proline hydroxylation and subsequent O-glycosylation of AZI1. Consistently, inhibition of prolyl hydroxylase reduces its apparent protein size. AZI1 secretion was examined using Arabidopsis protoplasts and seedling exudates. Employing Agrobacterium-mediated leaf infiltration of N. benthamiana, we attempted to assess long-distance movement of AZI1. In summary, the data point to AZI1 being a partially secreted protein and a likely new member of the group of hydroxyproline-rich glycoproteins. Its dual location suggests AZI1 to exert both intra- and extracellular functions.

Highlights

  • Plant hybrid proline-rich proteins (HyPRPs) are characterized by an N-terminal secretion signal peptide, a proline-rich domain (PRD) and a conserved eight-cysteine motif (8-CM) at the C-terminus

  • Given the consistent size pattern of AZI1-myc and AZI1-quintuple mutant derivatives one can conclude that AZI1 putative O-glycosylation and Mitogen-activated protein kinases (MAPKs)-targeted phosphorylation sites do not overlap, and that O-glycosylation unlikely occurs at residues Ser33, Ser41, Ser59, Thr66 or Thr70

  • Our data point to AZI1 being a hydroxyproline-rich glycoprotein

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Summary

Introduction

Plant hybrid proline-rich proteins (HyPRPs) are characterized by an N-terminal secretion signal peptide, a proline-rich domain (PRD) and a conserved eight-cysteine motif (8-CM) at the C-terminus. This latter hydrophobic C-terminal domain is found in lipid transfer proteins (LTPs), amylase inhibitors and 2S albumins [1,2]. Some tertiary structure data exists for 8-CM-containing proteins, though not for HyPRPs. a similar folding of the 8-CM region may be assumed [1]. Though the HyPRP sequence features are well-defined, little knowledge exists on protein functions and localization

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