Applications of Site-Specific Labeling to Study HAMLET, a Tumoricidal Complex of alpha-Lactalbumin and Oleic Acid

Background: Alpha-lactalbumin (a-LA) is a calcium-bound mammary gland-specific protein that is found in milk. This protein is a modulator of beta 1,4-galactosyltransferase enzyme, changing its acceptor specificity from N-acetyl-glucosamine to glucose, to produce lactose, milk's main carbohydrate. When calcium is removed from alpha-LA, it adopts a molten globule form, and this form, interestingly, when complexed with oleic acid (OA) acquires tumoricidal activity. Such a complex made from human alpha-LA (hLA) is known as HAMLET (Human A-lactalbumin Made Lethal to Tumor cells), and its tumoricidal activity has been well established. Methodology/Principal Findings: In the present work, we have used site-specific labeling, a technique previously developed in our laboratory, to label HAMLET with biotin, or a fluoroprobe for confocal microscopy studies. In addition to full length hLA, the alpha-domain of hLA (alpha D-hLA) alone is also included in the present study. We have engineered these proteins with a 17-amino acid C-terminal extension (hLA-ext and alpha D-hLA-ext). A single Thr residue in this extension is glycosylated with 2-acetonyl-galactose (C2-keto-galactose) using polypeptide-alpha-N-acetylgalactosaminyltransferase II (ppGalNAc-T2) and further conjugated with aminooxy-derivatives of fluoroprobe or biotin molecules. Conclusions/Significance: We found that the molten globule form of hLA and alpha D-hLA proteins, with or without C-terminal extension, and with and without the conjugated fluoroprobe or biotin molecule, readily form a complex with OA and exhibits tumoricidal activity similar to HAMLET made with full-length hLA protein. The confocal microscopy studies with fluoroprobe-labeled samples show that these proteins are internalized into the cells and found even in the nucleus only when they are complexed with OA. The HAMLET conjugated with a single biotin molecule will be a useful tool to identify the cellular components that are involved with it in the tumoricidal activity.

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