No Need To Be HAMLET Or BAMLET To Interact with Histones:  Binding of Monomeric α-lactalbumin to Histones and Basic Poly-amino Acids

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Fluorescence, Mathematical Methods, Molecules, Peptides and Proteins, Titration

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The ability of a specific complex of human α-lactalbumin with oleic acid (HAMLET) to induce cell death with selectivity for tumor and undifferentiated cells was shown recently to be mediated by interaction of HAMLET with histone proteins irreversibly disrupting chromatin structure [Duringer, C., et al. (2003) J. Biol. Chem. 278, 42131−42135]. Here we show that monomeric α-lactalbumin (α-LA) in the absence of fatty acids is also able to bind efficiently to the primary target of HAMLET, histone HIII, regardless of Ca2+ content. Thus, the modification of α-LA by oleic acid is not required for binding to histones. We suggest that interaction of negatively charged α-LA with the basic histone stabilizes apo-α-LA and destabilizes the Ca2+-bound protein due to compensation for excess negative charge of α-LA's Ca2+-binding loop by positively charged residues of the histone. Spectrofluorimetric curves of titration of α-LA by histone H3 were well approximated by a scheme of cooperative binding of four α-LA molecules per molecule of histone, with an equilibrium dissociation constant of 1.0 μM. Such a stoichiometry of binding implies that the binding process is not site-specific with respect to histone and likely is driven by just electrostatic interactions. Co-incubation of positively charged poly-amino acids (poly-Lys and poly-Arg) with α-LA resulted in effects which were similar to those caused by histone HIII, confirming the electrostatic nature of the α-LA−histone interaction. In all cases that were studied, the binding was accompanied by aggregation. The data indicate that α-lactalbumin can be used as a basis for the design of antitumor agents, acting through disorganization of chromatin structure due to interaction between α-LA and histone proteins.

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Biochemistry, v. 43, issue 19, p. 5575-5582