Membrane cholesterol modulates the dynamics and depth of penetration of κ-casein

Abstract

Aggregation of intrinsically disordered proteins (IDPs) is often triggered by their interaction with membranes, leading to several neurodegenerative diseases such as Parkinson’s, Alzheimer’s, Huntington, prion diseases and spinocerebellar ataxia. This calls for the detailed scrutiny of membrane interaction of IDPs with a special weightage on the intricate role of lipid composition on the protein dynamics. Generally, IDPs lack a well-ordered structure, however, capable of carrying out multiple biological functions because of their higher structural dynamism. In this work, we have chosen an IDP, κ-casein to evaluate its interaction with membranes with varying cholesterol content. Cholesterol plays a vital role in biology because of its involvement in various pathophysiological conditions and differential distribution in organelle membranes. We have deployed steady-state and time-resolved fluorescence spectroscopy to evaluate the dynamics and depth of penetration of κ-casein in the membrane with varying cholesterol composition. Our results demonstrate that the membrane cholesterol influences the depth of penetration and dynamics of κ-casein in the membrane. Taken together, our results reveal that κ-casein presumes higher order of tertiary structure in pure POPC membranes, and tryptophan is located at closer proximity nearer to the location of TMA-DPH (about 10.9 Å from the center of the bilayer). The tertiary structure of κ-casein loses rigidity in membranes containing cholesterol; however, the conformational rigidity is more than what it is in aqueous buffer. In short, we conclude steady-state and time-resolved fluorescence spectroscopy provides an exhaustive information on membrane-IDP interaction.