Expression of Human -Lactalbumin in Transgenic Tobacco

α-Lactalbumin (αLA), a major milk protein, is the regulatory subunit of lactose synthase. To assess the production of recombinant αLA in plants, the cDNAs for human αLA with or without its own signal sequence were introduced into tobacco plants under the control of the cauliflower mosaic virus 35S promoter. The gene integration and expression at the mRNA level were confirmed in several regenerated plants, while the expression at the protein level could be confirmed only in a transgenic tobacco transformed with the gene containing the signal sequence. The tobacco-expressed αLA migrated in SDS-PAGE with identical mobility to αLA prepared from human milk, indicating that the signal peptide of human αLA was correctly processed to yield a mature protein in tobacco plants. The expressed αLA (ca. 5 μg/g of fresh leaves) was found in the soluble fraction and eluted from a DEAE-Sepharose column in the same salt concentration range as the milk αLA. The partially purified tobacco-αLA was fully active in the synthesis of lactose when combined with galactosyltransferase. Thus, the transgenic tobacco produces a fully active mature αLA in a soluble form.