High-level secretion of a wheat lipid transfer protein in Pichia pastoris.

Abstract

Plant nonspecific lipid transfer proteins are small basic proteins with eight cysteine residues, all engaged in disulfide bonds. The sequence encoding the wheat 9-kDa LTP was cloned into the secretion vector pYAM7SP8 giving rise to pYTdltp4.90. Production in shake-flasks and a fermentor led to the synthesis of two major species of LTP: a larger than expected species of 14 kDa and a species of 10 kDa, close to the expected size of wheat LTP. When production was carried out in a fermentor with regulation of pH, oxygen level, and feed rate of carbon source, the 10-kDa species was the main protein at the end-point of culture. The recombinant wheat LTP (rLTP), secreted at a level of 720 mg/liter into the culture medium, is soluble. The rLTP was purified to homogeneity by ammonium sulfate precipitation, gel filtration, and anion-exchange chromatography, with a recovery yield of 36%. However, the molecular mass of rLTP, determined by mass spectrometry, is 9996 Da, while its naturally occurring counterpart has a molecular mass of 9607 Da. This discrepancy in size corresponds to a protein carrying three extra amino acids (DKR) at its N-terminal end, and this was confirmed by sequencing. In vitro lipid transfer activity showed that rLTP behaves in a similar way to the naturally occurring protein. These data indicate that Pichia pastoris is an efficient system for production of large quantities of soluble and biologically active rLTP for structure/function analysis.