Direct and indirect cholesterol effects on membrane proteins with special focus on potassium channels

As described in the literature the interaction between cholesterol and membrane proteins can occur via direct, ligand-like and indirect mechanisms, in which cholesterol effects are mediated by alterations in the biophysical properties or in the protein-organizing functions of the lipid membrane. Early studies emphasized the importance of indirect and raft-mediated effects, but improvements in computational and structural imaging techniques allowed the definition of a wide range of functionally active cholesterol binding domains and sites suggesting the relevance of direct cholesterol effects in various proteins. However, the intramolecular rearrangements induced by cholesterol leading to modulation of ion channel gating, membrane transport and receptor functions still have not been revealed. In this review we summarize the novel findings of the topic by focusing on recent studies about direct and indirect effects of cholesterol on potassium ion channels, and we extend the review to transporters and receptors with different domain structures to introduce the general mechanisms of cholesterol action among membrane proteins. We propose that rather than pure direct or indirect effects, cholesterol action on membrane proteins can be better described as a mixture of indirect and direct interactions with system-specific variability in their contributions, which can be explored by using a multi-level approach employing multiple experimental techniques.