During the last few years compelling evidence has been presented for the occurrence of cytoplasmic/nuclear plant lectins that are not detectable in normal plants but are only induced upon application of well-defined stress conditions. Since both the regulation of the expression and the subcellular location indicate that these 'non-classical lectins' are good candidates to play a physiologically important role as mediators of specific protein-carbohydrate-interactions within the plant cell, a critical assessment is made of the impact of these findings on the development of novel concepts about the role of plant lectins. Based on an analysis of the biochemical, molecular and evolutionary data of a jasmonate-induced chitin-binding lectin from tobacco leaves and a salt/jasmonate-induced leaf lectin from rice it is concluded that these lectins most probably interact with endogenous glycans located within the cytoplasmic/nuclear compartment of the plant cell. Several working mechanisms are proposed to explain how these inducible lectins may fulfill an important regulatory or structural role in stressed cells. In addition, the question of the evolutionary relationship(s) between the newly discovered inducible lectins and their 'classical' constitutively expressed homologs is addressed. Evidence is presented that the 'non-classical lectins' represent the main evolutionary line and that some of their corresponding genes were used as templates for genes encoding storage protein-like 'classical' homologs.
Searches in an EST database from maize revealed the expression of a protein related to the Galanthus nivalis (GNA) agglutinin, referred to as GNAmaize. Heterologous expression of GNAmaize in Pichia pastoris allowed characterisation of the first nucleocytoplasmic GNA homolog from plants. GNAmaize is a tetrameric protein which shares 64% sequence similarity with GNA. Glycan microarray analyses revealed important differences in the specificity. Unlike GNA, which binds strongly to high-mannose N-glycans, the lectin from maize reacts almost exclusively with more complex glycans. Interestingly, GNAmaize prefers complex glycans containing β1-2 GlcNAc residues. The obvious difference in carbohydrate-binding properties is accompanied by a 100-fold reduced anti-HIV activity. Although the sequences of GNA and GNAmaize are clearly related they show only 28% sequence identity. Our results indicate that gene divergence within the family of GNA-related lectins leads to changes in carbohydrate binding specificity, as shown on N-glycan arrays.
Molecular cloning of the “old” but still unclassified Euonymus europaeus agglutinin (EEA) demonstrated that the lectin is a homodimeric protein composed of 152 residue subunits. Analysis of the deduced sequence indicated that EEA is synthesized without a signal peptide and undergoes no posttranslational processing apart from the removal of a six-residue N-terminal peptide. Glycan array screening confirmed the previously reported high reactivity of EEA toward blood group B oligosaccharides but also revealed binding to high mannose N-glycans, providing firm evidence for the occurrence of a plant carbohydrate-binding domain that can interact with structurally different glycans. Basic Local Alignment Search Tool searches indicated that EEA shares no detectable sequence similarity with any other lectin but is closely related evolutionarily to a domain that was first identified in some abscisic acid- and salt stress-responsive rice (Oryza sativa) proteins, and, according to the available sequence data, might be ubiquitous in Spermatophyta. Hence, EEA can be considered the prototype of a novel family of presumably cytoplasmic/nuclear proteins that are apparently ubiquitous in plants. Taking into account that some of these proteins are definitely stress related, the present identification of the EEA lectin domain might be a first step in the recognition of the involvement and importance of protein-glycoconjugate interactions in some essential cellular processes in Embryophyta.
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