Coccinia indica agglutinin, a 17 kDa PP2 like phloem lectin: Affinity purification, primary structure and formation of self-assembled filaments

Abstract

Phloem protein-2 (PP2) is an abundant soluble protein in the sieve elements in plants. Its lectin property was reported in various species. The primary structure of a 17 kDa PP2 from Coccinia indica (Coccinia indica agglutinin, CIA17), determined by mass spectrometry, shows extensive homology with PP2 super family phloem lectins. Analysis of mass spectrometric data indicated the presence of 16 potential allelic variants of CIA17 with insignificant divergence in the primary structure. The primary structure contains an intramolecular disulfide bridge between Cys-34 and Cys-51, which is conserved across various cucurbit species and hence likely to be important for carbohydrate binding. CD spectroscopic studies revealed that CIA17 is rich in antiparallel β-sheets, similar to PP2 proteins from Cucurbita maxima and Arabidopsis thaliana. CD spectra recorded at various temperatures showed very little change in the spectral intensity and shape up to 90 °C, suggesting that CIA17 is a highly thermostable protein. Atomic force microscopic studies revealed that CIA17 forms filamentous structures at higher concentrations. In light of these results, we propose that CIA17 and other PP2 proteins play a role in the plant defense against pathogens by directly binding with the chitin cell wall, and also promote wound healing by forming self-assembled filaments.