Lipoprotein Particle Formation by Proapoptotic tBid
Vindana Ekanayake1, Danielle Nisan1, Pavel Ryzhov1
1Cancer Center, Sanford Burnham Prebys Medical Discovery Institute, La Jolla, California.
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 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.
Insights
The protein tBid forms tiny lipoprotein particles with phospholipids, which bind to the antiapoptotic protein Bcl-xL. This discovery reveals how lipids influence Bcl-2 protein interactions during apoptosis.
Area of Science:
- Biochemistry
- Cell Biology
- Molecular Biology
Background:
- Interactions between Bcl-2 family proteins and lipids regulate apoptosis, a key process in multicellular organisms.
- Bid and its cleaved form, tBid, are involved in mitochondrial outer membrane permeabilization and sequester antiapoptotic Bcl-2 proteins.
Purpose of the Study:
- To investigate the association of tBid with phospholipids.
- To characterize the resulting lipoprotein particles and their interaction with Bcl-xL.
Main Methods:
- Biochemical analysis of tBid-phospholipid interactions.
- Characterization of lipoprotein particle size and stoichiometry.
- Assessment of binding affinity between tBid lipoprotein particles and Bcl-xL.
Main Results:
- tBid associates with phospholipids to form soluble, nanometer-sized lipoprotein particles.
- These particles exhibit a lipid/protein stoichiometry of approximately 20/1 and a diameter of ~11.5 nm.
- Lipoparticle-bound tBid maintains its alpha-helical structure and binds Bcl-xL via its BH3 motif, forming a soluble complex.
Conclusions:
- Lipids play a crucial role in mediating the mobility and interactions of Bcl-2 proteins.
- The formation of tBid-lipoprotein particles provides a novel mechanism for regulating apoptosis.
- This study elucidates a new facet of Bcl-2 protein function in the context of lipid interactions.
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