Bcl-2 Family Proteins Regulate the Ability of Ceramide Channels to Permeabilize the Mitochondrial Outer Membrane to Proteins

Abstract

The sphingolipid, ceramide, forms channels capable of translocating proteins through membranes. These channels can form in the mitochondrial outer membrane at ceramide levels found to be present in that membrane early in apoptosis. For these channels to be good candidates for the protein release pathway that is a key, decision-making step in apoptosis, they need to be controlled by Bcl-2 family proteins. Full length Bcl-xL favors ceramide channel disassembly and activated, full length Bax favors the formation of large channels. These act at the low nanomolar level. Thus their mode of action on ceramide channels is consistent with their role in controlling the release of proteins from mitochondria. These proteins interact directly with the ceramide channels and are able to influence the size and stability of the channels whether the channels are formed on mitochondrial membranes or phospholipid membranes. The binding of Bcl-xL to the ceramide channel seems to form a 1:1 complex, the influence of the interaction propagating throughout the structure in an allosteric manner. The influence of Bax seems to involve multiple interactions, favoring a larger channel through an induced-fit mechanism. Alterations in either the protein structure or the ceramide structure can alter the interaction, providing clues to the site of interaction. Supported by a grant from NSF (MCB-0641208).