Production of correctly processed human serum albumin in transgenic plants

We have used a modified CaMV 35S promoter to direct the expression of chimaeric genes encoding human serum albumin (HSA) in transgenic potato and tobacco plants. To secrete the protein, either the human prepro-sequence or the signal sequence from the extracellular tobacco protein PR-S was used. We demonstrate secretion of HSA with both types of signal sequences in transgenic leaf tissue and in suspension cultures. HSA produced in transgenic potato plants was purified to chromatographic homogeneity. N-terminal amino acid sequence analysis revealed that the processing of the precursor protein was dependent on the type of signal sequence. Expression of the human preproHSA gene lead to partial processing of the precursor and secretion of proHSA. Fusion of HSA to the plant PR-S presequence resulted in cleavage of the presequence at its natural site and secretion of correctly processed HSA that is indistinguishable from the authentic human protein.