Shapes of Closed Phospholipid Membranes with Compartments

1. Shapes of Closed Phospholipid Membranes with Compartments Bojan Božič Institute of Biophysics, Faculty of Medicine, University of Ljubljana, Slovenia

2. Shape transformations of vesicles induced by β2-glycoprotein I Shapes of discoid intracellular compartments

3. Schematic representation of β2GPI Hydrophobic loop embedded in the membrane

4. Theory Thefree energy of the system G = W + Gp is the sum of the elastic energy of the phospholipid membrane and the free energy of the membrane bound proteins Gp = -εNp – kT(N0lnN0 – NplnNp – (N0 – Np)ln(N0 –Np)) ΔA0 = (N2 – N1)AL + NPAP

5. The equilibrium the shape equation KD = KD,0e(dW/dNp)/kT

6. Dependence of the number of buds on the relative volume and the concentration Rs = 20 µm = 15 µm = 10 µm

7. Shape deformations of a flaccid vesicle during injection of β2GPI

8. The time dependence of the number of buds during the injection of β2GPI β2GPI was injected in three periods of 150 s each.

9. Dependence of the number of buds on the relative volume and on the radius of a sphere with the same membrane area (6) (6) (9) (19) (22) Rs [µm] Experimentally acquired data from vesicles A to E are positioned on the diagram.

10. The effect of the gravity A larger β2GPIconcentration is needed to produce the same degree of budding.

11. The dependence of the radius of a vesicle at a rim on the number of buds

12. Conclusions • The extent of budding is an increasing function of β2GPI concentration. • The budding can be rationalized by assuming that part of β2GPI is inserted into the outer leaflet of the bilayer. • The contribution to ΔA0 of each bound β2GPI was estimated to be about one tenth of the area of the cross-section of its inserted portion. • The greater number of buds is characteristic of more flaccid and larger vesicles. • Different vesicles behave differently because the neck between the main vesicle body and buds or strings of buds can be either closed or open.

13. Shapes of intracellular compartments The discoid shape can be stabilized by adhesion in the central part,weak lateral segregation of mobile membrane constituents andformation of stiffer membrane regions with a defined spontaneous curvature.

14. Shape features of discoid compartments with homogeneous membrane v = 0.2 Phase diagram describingthe values of vand N at which the membrane comes into contact. v

15. 1000 adhesion molecules per µm2 Shape changes due to adhesion in the central discoid partLateral segregation of membrane constituents

16. Shapes with two distinct membrane regions The stiffer membrane regions are presented by red lines and the soft membrane regions by black lines.

17. c0=C0Rs

19. EM micrographs of fusiform vesicles (FV) in urothelial umbrella cells The plaque regions of FVs arehighlighted by red lines and the hinge regions by blue lines. 1 µm 100 nm Apical region of an umbrella cell with numerous FVs.

20. Conclusions • A noticeable effect on the shape can be realized even when the local membrane composition does not vary considerably across the membrane. • The spontaneous curvature of protein scaffolds supporting the rim may be even more important than their relative stiffness. • The stiffness of plaques is at least an order of magnitude larger than the stiffness of bare membrane. • All three scenarios can lead to qualitatively similar shapes with a flattened central part and a drop-like cross-section at the rim. •At certain values of model parameters the three scenarios can also yield different shapes.

21. Saša Svetina Jure Derganc Jasna Prebil Janja Majhenc Gregor Gomišček Veronika Kralj-Iglič Rok Romih (Institute of Cell Biology, Faculty of Medicine, Ljubljana) Jure Stojan (Institute of Biochemistry, Faculty of Medicine, Ljubljana) BlažRozman (Department of Rheumatology, University Medical Centre, Ljubljana)