Barry Lentz, PhD

RESEARCH INTERESTS:

Biomembrane Microstructure and Cell Function

Our research seeks to reveal biomembrane structural features leading to two aspects of membrane function: the role of platelet membranes in blood coagulation and the involvement of bilayer microstructures in cell membrane fusion.

The activation of prothrombin to thrombin (a key serine protease and regulatory molecule in blood coagulation) requires Ca2+, activated factor X enzyme (X_a), and activated factor V cofactor (V_a). These components bind to negatively charged phospholipid membranes (probably supplied, in vivo, by platelet membranes) to form the "prothrombinase". This complex enzyme catalyzes activation of prothrombin to thrombin many thousand times faster than factor X_a alone. Our research aims to understand how the platelet membrane achieves this essential rate enhancement. We propose that interactions of phosphatidylserine [PS] with specific sites on prothrombin, factor X_a and factor V_a trigger conformational changes that efficiently and sequentially direct to the active site of factor X_a the two peptide bonds that must be hydrolyzed by this protease. Since PS is located on the inner leaflet of resting platelet membranes and platelet activation exposes it to the plasma, PS becomes a second messenger in regulating blood coagulation. Our current research combines enzymological and biophysical approaches for testing several aspects of this hypothesis.

Endocytosis, excretory processes, cell division, and membrane biogenesis are some of the cell functions that depend on membrane fusion. While the overall process is certainly controlled by cellular proteins, the molecular mechanisms by which the essential lipid rearrangements are accomplished are very poorly understood. Our second project, then, aims to define the particular membrane microstructures that allow fusion of model membrane vesicles brought into close apposition by a dehydrating polymer, poly(ethylene glycol) [PEG]. The advantage of this model system is its simplicity relative to the complexity of a cellular system. Our approach has been to identify and characterize the microstructural features common to membranes that fuse when aggregated by PEG. Thus far, we have shown that disruption of lipid packing in contacting monolayers is necessary for fusion. We have identified a three-step process by which fusion proceeds, and have shown that biomembrane fusion likely follows the same process. Our emphasis now is on understanding the molecular details of this process and how the fusion peptides common to most biological fusion proteins might facilitate this process. The results are already suggesting mechanisms by which membrane fusion may be controlled in vivo. A proposed mechanism for model and biomembrane fusion. First order rates k₁ and k₂ and "pop" rates [1/(sÖ 2p )] have been determined for model membranes at several temperatures.

RECENT PUBLICATIONS:

Majumder R, Quinn-Allen MA, Kane WH, Lentz BR. A phosphatidylserine binding site in factor Va C1 domain regulates both assembly and activity of the prothrombinase complex.Blood. 2008 Jun 27.

Weinreb G, Lentz BR. Analysis of membrane fusion as a two-state sequential process: evaluation of the stalk model. Biophys J. 2007 Jun 1;92(11):4012-29.

Lentz BR. PEG as a tool to gain insight into membrane fusion. Eur Biophys J. 2007 Apr;36(4-5):315-26. Review.

Lentz BR. Seeing is believing: the stalk intermediate. Biophys J. 2006 Oct 15;91(8):2747-8.

Majumder R, Weinreb G, Lentz BR. Efficient thrombin generation requires molecular phosphatidylserine, not a membrane surface. Biochemistry. 2005 Dec 27;44(51):16998-7006.

Dennison SM, Bowen ME, Brunger AT, Lentz BR. Neuronal SNAREs do not trigger fusion between synthetic membranes but do promote PEG-mediated membrane fusion. Biophys J. 2006 Mar 1;90(5):1661-75.

Haque ME, Koppaka V, Axelsen PH, Lentz BR. Properties and structures of the influenza and HIV fusion peptides on lipid membranes: implications for a role in fusion. Biophys J. 2005 Nov;89(5):3183-94.

Majumder R, Quinn-Allen MA, Kane WH, Lentz BR. The phosphatidylserine binding site of the factor Va C2 domain accounts for membrane binding but does not contribute to the assembly or activity of a human factor Xa-factor Va complex. Biochemistry. 2005 Jan 18;44(2):711-8.