Lipoprotein Particle Formation by Proapoptotic tBid

Abstract

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 a-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.

Apoptotic mitochondrial poration by a growing list of pore‐forming BCL‐2 family proteins

“…slow tumbling of the tBID remainder in the lipoprotein assembly. [68] It is possible that tBID interacts with opened bilayers by lining the apoptotic pore lumen like the apoptotic dimers of BAK and BAX (described below). Overall, monomeric tBID likely destabilizes membranes by…”

“…[ 67 ] Remarkably, tBID has also been shown to form lipoparticles (nanodiscs) with an average diameter of 11.5‐nm (at 20:1 lipid:protein ratio) exhibiting a broad size distribution according to SEC analysis, and with a composition of 56% α‐helix and 6% β‐strand based on CD spectroscopy and with only 30–40% of the amide signals out of 136‐residues visible in 1 H/ 15 N 2D NMR suggesting intrinsic disorder in the NMR visible regions as well as slow tumbling of the tBID remainder in the lipoprotein assembly. [ 68 ] It is possible that tBID interacts with opened bilayers by lining the apoptotic pore lumen like the apoptotic dimers of BAK and BAX (described below). Overall, monomeric tBID likely destabilizes membranes by insertion in an open loosely associated helical conformation that can further organize with additional nearby monomers to generate and line the pore lumen forming heterogeneous proteolipidic pores that remain to be characterized structurally (Figure 2E, Table 2).…”