Structure-function analysis of HAMLET (human alpha-lactalbumin made lethal to tumor cells)

Abstract: The human genome sequence encodes fewer proteins than expected, suggesting that one protein can have several functions and adjust their structure to meet different structural demands. Changes in tertiary structure have mostly been associated with disease and the most striking example is the prion protein, which changes from a mixed alpha-helical and beta-sheet conformation to the beta-sheet rich, disease causing iso-form. Protein folding is becoming recognised as a mechanism to generate beneficial functional diversity, however. One example is alpha-lactalbumin, which by unfolding forms a tumoricidal complex with oleic acid called human alpha-lactalbumin made lethal to tumor cells (HAMLET), which kills tumor cells. We investigated how protein sequence variation influences the formation of HAMLET, by comparing human, bovine, equine, caprine and porcine alpha-lactalbumin (paper I). The fatty acid specificity in HAMLET was studied using fatty acids differing in chain lenth, saturation and orientation of the double bond (paper II). We have also used stably unfolded alpha-lactalbumin mutants (paper III) to exclude effects of the native protein in tumor cells. In an attempt to determine the three-dimensional structure of HAMLET (paper IV) crystals were characterised by X-ray diffraction at 1.8 Å resolution. The unit cell differed significantly from native alpha-lactalbumin, confirming that the protein had undergone major structural alterations to form HAMLET. These studies have added significantly to our understanding of HAMLET as a tumoricidal molecular complex. It is possible that HAMLET is only one of many beneficial protein-folding variants waiting to be discovered.