Can misfolded proteins be beneficial? The HAMLET case

Abstract

By changing the three-dimensional structure, a protein can attain new functions, distinct from those of the native protein. Amyloid-forming proteins are one example, in which conformational change may lead to fibril formation and, in many cases, neurodegenerative disease. We have proposed that partial unfolding provides a mechanism to generate new and useful functional variants from a given polypeptide chain. Here we present HAMLET (Human Alpha-lactalbumin Made LEthal to Tumor cells) as an example where partial unfolding and the incorporation of cofactor create a complex with new, beneficial properties. Native alpha-lactalbumin functions as a substrate specifier in lactose synthesis, but when partially unfolded the protein binds oleic acid and forms the tumoricidal HAMLET complex. When the properties of HAMLET were first described they were surprising, as protein folding intermediates and especially amyloid-forming protein intermediates had been regarded as toxic conformations, but since then structural studies have supported functional diversity arising from a change in fold. The properties of HAMLET suggest a mechanism of structure-function variation, which might help the limited number of human protein genes to generate sufficient structural diversity to meet the diverse functional demands of complex organisms.

Structural Origin of ELOA Toxicity – Implication for HAMLET-Type Protein Complexes with Oleic Acid

“…Such mutant formed a tumoricidal HAMLET-like complex with OA, but the partially unfolded protein alone did not kill tumor cells. Another non-native -lactalbumin variant with all amino acids building disulfide bridges substituted by Ala residues also did not exhibit cytotoxic activity in the absence of OA, while its HAMLET-like form displayed strong tumoricidal activity against lymphoma and carcinoma cell lines [13]. Together, these experiments consistently confirmed that both molten globule like protein conformation and specific fatty acids are required for the tumoricidal activity of the investigated complexes.…”

α-Lactalbumin, Engineered to be Nonnative and Inactive, Kills Tumor Cells when in Complex with Oleic Acid: A New Biological Function Resulting from Partial Unfolding

“…Hence, unlike amyloid fibril examples, deliberately engineered, nonnative, partially unfolded structural ensembles may surprisingly confer cytotoxic effects selectively to tumor cells or, in other words, independent beneficial effects to the host cells. 40 In light of the 100% authentic population of the molten globule state with rHLA all-Ala and rHLA all-Ala -OA, we are able to speculate the nature of the small native-like population contained within HAMLET. In other words, does HAMLET partition into fatty acid-bound molten globule molecules and fatty-acid-free native molecules, or are the molecules exhibiting native-like signatures (CD and NMR; Figs.…”

“…While it is becoming evident that biological activity can be maintained even after partial unfolding of a protein 38 and a considerable proportion of eukaryotic proteins are known to possess biological activity in their unstructured states, 39 the example of α-lactalbumin and HAMLET shows that a protein can exhibit significantly distinct biological functions depending on whether it is in the native or partially unfolded state. 40 In this paper, we provide insight into how the lack of native threedimensional structure in the α-lactalbumin protein positively contributes to the selective in vivo tumoricidal activity of the complex.…”

An Uncommon Presentation of Amyloidosis

“…Sixty different types of amyloid protein have been identified so far, and thirty-six have been connected with deposition disease. 7,8 For example, AA amyloidosis is caused by the deposition of an acute phase reactant protein in various organs. This acute phase reactant is usually generated in the setting of inflammatory disease (inflammatory bowel disease, rheumatoid arthritis, etc) or chronic infection (tuberculosis, chronic osteomyelitis, etc).…”