Proteins isolated from lucerne roots by affinity chromatography with sugars analogous to Nod factor moieties

Abstract

Nod factors are important elicitors in legume-bacterium symbiosis. Any candidate plant receptor(s) for these lipo-oligosaccharides can be expected to show some lectin-like properties. A novel protein (P60), a native tetramer with 60 kDa monomers, has been isolated from a membrane fraction of Medicago sativa (lucerne, alfalfa) roots by using affinity chromatography with either GlcNAc or N,Nʹ,Nʹʹ-triacetyl-(1 → 4)-β-D-chitotriose [(GlcNAc)3] grafted to agarose beads as the matrix and, in a second step, Sephadex G-200 gel filtration. With (GlcNAc)3-agarose an additional protein of 78 kDa was isolated. P60 showed haemagglutination activity with specificity for GalNAc, GalN, GlcNAc and GlcN. Binding experiments with radioactive GlcNAc gave a Kd of 95 nM and one binding site per monomer of P60; Nod factor competed strongly for this binding. In native PAGE, protein incubated with O-sulphated Nod factors had a higher electrophoretic mobility as a consequence of binding. However, the largest modification was observed with a natural mixture of Nod factors, containing the O-acetylated and O-sulphated tetrasaccharidic NodRm-IV(Ac,S) (in which Ac stands for an O-acetylated group at the non-reducing end and S for O-sulphation at the reducing end) in addition to the non-O-acetylated NodRm-IV(S) (which alone had little effect) and NodRm-V(S). The native PAGE study was also performed with known lectins from other sources, but only the 34 kDa lectin of Phytolacca americana (pokeweed) showed any such interaction, although without discrimination between Nod factors. Finally, one peptide of each isolated protein was sequenced; the peptide from P60 showed some similarity with dihydrolipoamide dehydrogenase and ferric leghaemoglobin reductase, whereas the peptide from P78 was identical with an analogous region of 70 kDa heat shock protein.