Interactions between plant lipid-binding proteins and their ligands

Lipids participate in diverse biological functions including signal transduction, cellular membrane biogenesis and carbon storage. Following de novo biosynthesis in the plastids, fatty acids (FAs) are transported as acyl-CoA esters to the endoplasmic reticulum where glycerol-3-phosphate undergoes a series of acyl-CoA-dependent acylation via the Kennedy pathway to form triacylglycerols for subsequent assembly into oils. Alternatively, newly synthesized FAs are incorporated into phosphatidylcholine (PC) by a PC:acyl-CoA exchange process defined as “acyl editing”. Acyl-CoA-binding proteins (ACBPs) at various subcellular locations can function in lipid transfer by binding and transporting acyl-CoA esters and maintaining intracellular acyl-CoA pools. Widely distributed in the plant kingdom, ACBPs are found in all eukaryotes and some eubacteria. In both rice and Arabidopsis, six forms of ACBPs co-exist and are classified into four groups based on their functional domains. Their conserved four-helix structure facilitates interaction with acyl-CoA esters. ACBPs also interact with phospholipids as well as protein partners and function in seed oil regulation, development, pathogen defense and stress responses. Besides the ACBPs, other proteins such as the lipid transfer proteins (LTPs), annexins and lipid droplet-associated proteins are also important lipid-binding proteins. While annexins bind Ca²⁺ and phospholipids, LTPs transport lipid molecules including FAs, acyl-CoA esters and phospholipids.