HOMOLOGY MODELING AND MOLECULAR-DYNAMICS AIDED ANALYSIS OF LIGAND COMPLEXES DEMONSTRATES FUNCTIONAL-PROPERTIES OF LIPID-TRANSFER PROTEINS ENCODED BY THE BARLEY LOW-TEMPERATURE-INDUCIBLE GENE FAMILY, BLT4

The homology modelling technique was used to predict the tertiary stru ctures of three members of the low-temperature-inducible barley vegeta tive shoot epidermal lipid-transfer protein (LTP) family, BLT4, on the basis of the X-ray crystallographically determined three-dimensional structure of a maize seedling LTP. Differences between the maize LTP a nd the BLT4 family include amino acid substitutions around the entranc e and inside the predicted hydrophobic binding tunnels of these protei ns. Because of the deletion of the loop region corresponding to Val60- Gly62 of the maize LTP from all three BLT4 LTPs, their internal hydrop hobic tunnels are longer. Molecular dynamics modelling shows that BLT4 .9 can accommodate hexadecanoic acid in its binding tunnel in similar conformation to the maize LTP. However, modelled cis,cis-9,12-octadeca ndienoic acid had a more favourable interaction with the BLT4.9 LTP th an with the maize protein. Di-cis,cis-9,12-octadecandienoyl phosphatid ylglycerol and di-cis,cis-9,12-octadecandienoyl phosphatidylcholine we re modelled in the BLT4.9 structure with the fatty acyl group at posit ion 1 embedded in the binding tunnel and the group at position 2 locat ed on the solvent accessible surface of the protein. The results of th e modelling suggest that the phospholipid headgroup can form hydrogen and salt bridges with polar and charged residues outside the binding t unnel and the exposed hydrocarbon chain interacts with hydrophobic ami no acids on the surface. These results are consistent with the proposa l that BLT4 LTPs have a lipid-transfer function associated with frost acclimation in barley.