The biochemistry and biology of extracellular plant lipid‐transfer proteins (LTPs)

Abstract

Plant lipid-transfer proteins (LTPs) are abundant, small, lipid binding proteins that are capable of exchanging lipids between membranes in vitro. Despite their name, a role in intracellular lipid transport is considered unlikely, based on their extracellular localization. A number of other biological roles, including antimicrobial defense, signaling, and cell wall loosening, have been proposed, but conclusive evidence is generally lacking, and these functions are not well correlated with in vitro activity or structure. A survey of sequenced plant genomes suggests that the two biochemically characterized families of LTPs are phylogenetically restricted to seed plants and are present as substantial gene families. This review aims to summarize the current understanding of LTP biochemistry, as well as the evidence supporting the proposed in vivo roles of these proteins within the emerging post-genomic framework.

The Formation and Function of Plant Cuticles

“…These proteins represent a unique class of LTPs, a family of small and typically soluble proteins that bind a variety of lipid substrates in vitro (Yeats and Rose, 2008). A major remaining question is how hydrophobic cuticle precursors are transported across the hydrophilic environment of the polysaccharide cell wall to the cuticle.…”

“…A major remaining question is how hydrophobic cuticle precursors are transported across the hydrophilic environment of the polysaccharide cell wall to the cuticle. Apoplastic LTPs have been proposed to play a role, although genetic or biochemical evidence for their involvement in transport is generally lacking (Yeats and Rose, 2008). In the case of the dihydroxyacyl cutin precursor 2-MHG, the glycerol moiety imparts sufficient polarity to allow aqueous solubility at low millimolar concentrations (Yeats et al, 2012b).…”

On the role of a Lipid-Transfer Protein. Arabidopsis ltp3 mutant is compromised in germination and seedling growth.

“…[1][2][3][4][5] Unlike their animal counterparts, plant LTPs typically have broad substrate specificity and are also named nonspecific LTPs. Their affinity for lipids is presumed to be crucial for their biological function, but in vivo substrates remain largely unknown.…”

Arabidopsis LTPG Is a Glycosylphosphatidylinositol-Anchored Lipid Transfer Protein Required for Export of Lipids to the Plant Surface

“…Lipid transfer proteins (LTPs) were proposed as candidates that played a role in the delivery of wax components during the assembly of the cuticle (Sterk et al, 1991;Yeats and Rose, 2008). Originally described as proteins that facilitate lipid transfer between liposomes in vitro (Kader et al, 1984), LTPs possess several characteristics that make them suitable for delivering wax to the cuticle: they have a hydrophobic cavity (Kader, 1996); they are capable of binding fatty acids in vitro, and binding is impaired upon disruption of the hydrophobic pocket (Zachowski et al, 1998); and they are extracellular proteins and at <25 kD (Beisson et al, 2003) are likely small enough to fit through the pores of the plant cell wall (Baron-Epel et al, 1988).…”