Mechanisms of Action of Plant Proteins That Bind and Transfer Key Regulatory Hydrophobic Compounds

To perform their numerous functions in plants, hydrophobic substances, such as lipids and some hormones, must be transferred from sites of synthesis to their destination through an aqueous environment. This process is facilitated by proteins possessing a cavity lined with non-polar radicals that bind and retain hydrophobic compounds. These proteins are the Lipid Binding and Transfer Proteins (LBTPs). Although much is known about the structure and ligand affinity of animal LBTPs this review summarizes evidence that some classes of plant LBTPs are functionally homologous to those of animals. We describe how LBTPs contribute to building the functional structure of lipids as well as to signaling by lipids and plant hormones including the long-distance transport. We also consider LBTP involvement in the deposition of suberin and cutin, which protects plants from pathogen and abiotic stresses amongst other processes. Most proposed functions of LBTPs are based on their ability to bind various hydrophobic substance in vitro, on experimentally established co-localizations of LBTPs and their putative ligands in the cells and on the isolation from plants of complexes of these proteins with hydrophobic ligands. Although many aspects of plant LBTPs action remain unclear the present review highlights the hitherto widely overlooked significance of these plant proteins and a pressing need for their further study.