Lipoprotein Particle Formation by Proapoptotic tBid.

The interactions of Bcl-2 family proteins with intracellular lipids are essential for the regulation of apoptosis, a mechanism of programmed cell death that is central to the health and development of multicellular organisms. Bid and its caspase-8 cleavage product, tBid, promote the permeabilization of the mitochondrial outer membrane and sequester antiapoptotic Bcl-2 proteins to counter their cytoprotective activity. Bid and tBid also promote lipid exchange, a characteristic trait of apoptosis. Here, we show that tBid is capable of associating with phospholipids to form soluble, nanometer-sized lipoprotein particles that retain binding affinity for the antiapoptotic protein Bcl-xL. The tBid lipoprotein particles form with a lipid/protein stoichiometry in the range of 20/1 and have a diameter of ∼11.5 nm. Lipoparticle-bound tBid retains an α-helical structure and binds Bcl-xL through its third Bcl-2 homology motif, forming a soluble, lipid-associated heteroprotein complex. The results shed light on the role of lipids in mediating Bcl-2 protein mobility and interactions.