Unifying the two different classes of plant non-specific lipid-transfer proteins allergens classified in the WHO/IUIS allergen database through a motif with conserved sequence, structural and electrostatic features

Abstract

The ubiquitously occuring non-specific lipid-transfer proteins (nsLTPs) in plants are implicated in key processes like biotic and abiotic stress, seed development and lipid transport. Additionally, they constitute a panallergen multigene family present in both food and pollen. Presently there are 49 nsLTP entries in the WHO/IUIS allergen database (http://allergen.org/). Analysis of full-length allergens identified only two major classes (nsLTP1,n=32 and nsLTP2,n=2), although nsLTPs are classified into many other groups. nsLTP1 and nsLTP2 are differentiated by their sequences, molecular weights, pattern of the conserved disulphide bonds and volume of the hydrophobic cavity. The conserved R44 is present in all full length nsLTP1 allergens (only Par j 2 from Parietaria judaica has K44), while D43 is present in all but Par j 1/2 from P. judaica (residue numbering based on PDBid:2ALGA). Although, the importance of these residues is well-established in nsLTP1, the corresponding residues in nsLTP2 remain unknown. A structural motif comprising of two cysteines with a disulphide bond (C3-C50), R44 and D43 identified a congruent motif (C3/C35/R47/D42) in a nsLTP2 protein from rice (PDBid:1L6HA), using the CLASP methodology. This also provides a quantitative method to assess the cross-reactivity potential of different proteins through congruence of an epitope and its neighbouring residues. Future work will involve obtaining the PDB structure of an nsLTP2 allergen and Par j 1/2 nsLTP1 sequences with a missing D43, determine whether nsLTP from other groups beside nsLTP1/2 are allergens, and determine nsLTP allergens from other plants (chickpea, walnut, etc.) based on a genome wide identification of genes with conserved allergen features.