The Carrot Extracellular Lipid Transfer Protein EP2: Quantitative Aspects With Respect to its Putative Role in Cutin Synthesis

Plant lipid transfer proteins (LTPs) have been isolated from different sources in both monocot and dicot plant species (see for review Kader, 1990). The purified proteins were shown to be small basic proteins capable of transferring several types of lipids between various types of membranes in vitro. Based on this observation, it was suggested that, analogous to cytosolic LTPs from mammals (Wirtz, 1991), the transfer of phospholipids between organelle and endoplasmatic reticulum membrane systems would be their function in vivo (Arondel and Kader, 1990; Kader, 1990). However, the presence of a putative signal peptide sequence in mRNAs of previously identified plant LTPs (Bernhard et al., 1991; Skriver et al., 1992; Tchang et al., 1988), as well as of putative LTPs identified on the basis of cDNA-derived amino acid sequence homology (Bernhard and Sommerville, 1989; Fleming et al., 1992; Foster et al., 1992; Hughes et al., 1992; Sterk et al., 1991; Torres-Schumann et al., 1992; Weig et al., 1992), indicated that they most likely represent secreted proteins. In barley, an LTP previously identified as an α-amylase/protease inhibitor, was found to be present in the medium of an aleurone cell culture (Bernhard and Sommerville, 1989; Mundy and Rogers, 1986). Similarly, immunological studies on a putative LTP in Arabidopsis revealed that the protein is present extracellularly in cell walls of epidermal cells (Thoma et al., 1993). These results indicate that plant LTPs are extracellular proteins. Consequently, their function needs to be readdressed.