Impact of self-association on exchangeable apolipoproteins' structure and function
How does exchangeable apolipoproteins’ self-association impact their function?
Our hypothesis is that lipid-free exchangeable apolipoproteins’ self-association affects the protein function in two ways: (1) it regulates lipid-free exchangeable apolipoproteins’ biological reactivity (e.g lipid binding, cell membrane interaction, enzyme binding) and (2) it protects against detrimental modifying reactions (e.g. oxidation, glycosylation, crosslinking).
The experimental approach
(1) The more tightly the apolipoprotein binds to other homologous protein molecules, the more difficult is for lipids to disrupt those interactions and bind to the apolar regions of the protein. We and other laboratories have demonstrated in vitro that an increase in the levels of apoA-I self-association inhibits apoA-I lipid-binding and ATP binding cassette A1-mediated (ABCA1) lipid release from cells to apoA-I.
Using protein mutagenesis, chromatographies, circular dichroism and fluorescence spectroscopies, mass-spectrometry and cell culture techniques, we are investigating the hypothesis that in vivo modifications of apoA-I, including specific residue oxidation, crosslinking and natural mutations, may change the protein function (e.g. lipid-binding, ABCA1-mediated cellular lipid release) by altering the degree of self-association or the stability of specific self-associated species.
(2) Several natural apoA-I mutants form amyloid fibrils (normally composed of N-terminal fragments of these proteins) that accumulate in various organ with sometime lethal consequences (hereditary amyloidosis). An increasing number of reportsindicate that amyloid deposits are also a significant component of the atherosclerotic plaque. Full-length and N-terminal fragments of wild-type apoA-I make up a large part of these deposits (non-hereditary amyloidosis). In vitro it has been demonstrated that oxidative modifications of full-length apoA-I can generate amyloidogenic variants of the protein. These oxidation reactions are similar to those occurring in the highly oxidative milieu that is promoted by inflammation in the sub-endothelial space of atherosclerotic arteries.
We are interested in studying atherosclerosis-relevant mechanisms whereby wild-type apoA-I becomes amyloidogenic. To elucidate the general mechanism of amyloid formation by apolipoproteins, we are investigating the pathway(s) that are involved in the fibrillation of apoA-I in hereditary and non-hereditary amyloidoses. In particular, we want to determine if different levels of self-association, whether due to natural mutations or chemical modifications of apoA-I, can alter the susceptibility of apoA-I to oxidation and amyloid fibrils formation.
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