Professor of Biochemistry & Biophysics
PHD - Cornell University
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 (Xa), and activated factor V cofactor (Va). 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 Xa 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 Xa and factor Va trigger conformational changes that efficiently and sequentially direct to the active site of factor Xa 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.
- Majumder, R, Koklik, T, Rezaie, AR, and Lentz, BR: Phosphatidylserine-induced factor Xa dimerization and binding to factor Va are competing processes in solution. Biochemistry, 52, 143-51, 2013.
- Chakraborty, H, Tarafdar, Klapper, DG, and Lentz, BR: Effects of Wild Type and Mutant HA Fusion Peptides on Kinetics and Activation Thermodynamics of Stalk and Pore Formation: Mechanistic Implications. Biophysical Journal, 103, submitted, 2013.
- Tenchov, B.G., MacDonald, R.C., and Lentz, B.R. Fusion Peptides Promote Formation Of Bilayer Cubic Phases In Lipid Dispersions. An X-Ray Diffraction Study. Biophysical Journal, 103, 1029-37, 2013.
- Tarafdar, P.K., Chakraborty, H., Dennison, S. Moses and Lentz, B.R.: Phosphatidylserine Inhibits and Calcium Promotes Model Membrane Fusion. Biophysical Journal, 103, 1880-89, 2013.
- Chakraborty, H, Tarafdar, PK, Bruno, MJ, Sengupta, T and Lentz, BR: Activation Thermodynamics of PEG-Mediated Model Membrane Fusion Support Mechanistic Models of Stalk and Pore Formation. Biophysical Journal, 102, 2751-2760, 2012.
- Chakraborty, H and Lentz, BR: A simple method for correction of circular dichroism spectra obtained from membrane-containing samples. Biochemistry, 51, 1005-8, 2012.
- Majumder, R, Liang, X, Quinn-Allen, MA, Lentz, BR, and Kane, WH: Modulation of prothrombinase assembly and activity by phosphatidylethanolamine. JBC, 286, 35535-35542, 2011.
- Haque, MdE, Chakraborty, H, Koklic, T, Komatsu, H, Axelsen, PH, and Lentz, BR: Hemagglutinin Fusion Peptide Mutants in Model Membranes: Structural Properties; Membrane Physical Properties and PEG-Mediated Fusion, Biophysical Journal, 101, 1095-1104, 2011.
- Koklic, T, Majumder, R., Weinreb, G., and Lentz, B.R. Factor Xa Binding to Phosphatidylserine-Containing Membranes Produces an Inactive Membrane-Bound Dimer, Biophysical Journal, 97, 2232-2241, 2009.
- Chattopadhyay, R, Iacob, R, Majumder, R, Sen, S, Tomer, K, and Lentz, BR: Functional and Structural Characterization of Factor Xa Dimer in Solution, Biophysical Journal, 96, 974-986, 2009.
- Majumder, R, Quinn-Allen, MA, Kane, WH and Lentz, BR: A Phosphatidylserine Binding Site in Factor Va C1 Domain Regulates both Assembly and Activity of the Prothrombinase Complex. Blood, 112, 2795-802, 2008.
- 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.
- Weinreb, G. and Lentz, BR: Analysis of Membrane Fusion as A Two-State Sequential Process: Evaluation of The Stalk Model. Biophysical Journal, 92, 4012-4029, 2007.
- Lentz, BR: PEG as a Tool to Gain Insight into Membrane Fusion. Eur. J. Biophysics, 36, 315-326, 2007.
- 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.
- 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, Weinreb G, Lentz BR. Efficient thrombin generation requires molecular phosphatidylserine, not a membrane surface. Biochemistry. 2005 Dec 27;44(51):16998-7006.
- 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.
3044 Genetic Medicine Bldg
Campus Box 7260
Chapel Hill, NC 27599