Plant lipid transfer proteins: are we finally closing in on the roles of these enigmatic proteins?

Abstract

The nonspecific lipid transfer proteins (LTPs) are small compact proteins folded around a tunnel-like hydrophobic cavity, making them suitable for lipid binding and transport. LTPs are encoded by large gene families in all land plants, but they have not been identified in algae or any other organisms. Thus, LTPs are considered key proteins for plant survival on and colonization of land. LTPs are abundantly expressed in most plant tissues, both above and below ground. They are usually localized to extracellular spaces outside the plasma membrane. Although the in vivo functions of LTPs remain unclear, accumulating evidence suggests a role for LTPs in the transfer and deposition of monomers required for assembly of the waterproof lipid barriers, such as cutin and cuticular wax, suberin, and sporopollenin, formed on many plant surfaces. Some LTPs may be involved in other processes, such as signaling during pathogen attacks. Here, we present the current status of LTP research with a focus on the role of these proteins in lipid barrier deposition and cell expansion. We suggest that LTPs facilitate extracellular transfer of barrier materials and adhesion between barriers and extracellular materials. A growing body of research may uncover the true role of LTPs in plants.

Keywords: LTP; allergen; biopolymer; cell wall; cuticle; suberin.

Fig. 1.

The 3D fold of wheat LTP1 [renamed TaLTP1.1 in (2)] structure [PDB ID 1GH1 (3)]. The four helices are stabilized by four disulfide bridges (green sticks) of which the first and fourth bond connect the N- and C-terminal parts to H3, respectively. The second and third bond link H2 to H1 and H4. The hydrophobic central cavity of the LTP1 structure is formed by residues (brown sticks) from each helix and the unstructured C-terminal part.

Fig. 2.

The size and shape of the ligand binding cavity of wheat LTP1 [renamed TaLTP1.1 in (2)] without a ligand [PDB ID 1GH1 (3)] (A), complexed with PGB₂ [PDB ID 1CZ2 (83)] (B) and complexed with two LMPC molecules [PDB ID 2BWO (15)] (C). The size of the cavity (shown as white wireframe) is adjusted by the size of the bound ligand by the movement of the C-terminal residues shown as sticks.

Fig. 3.

Putative roles for different LTPs during lipid barrier assembly in green tissues (A), roots (B), and pollen (C). Three different roles for LTPs in lipid barrier assembly are indicated. LTPg (orange) is attached to the plasma membrane through its GPI-anchor and dock the lipid barrier monomers when they leave the plasma membrane-localized ABC transporters. The Transporter LTPs (purple) facilitate the transfer of the lipid barrier monomers from the LTPg to the site of barrier deposition. The Adhesion LTPs (yellow) have a structural role adhering the hydrophobic barrier to the hydrophilic cell wall (A, B) or intine (C).