The spontaneous curvature of membranes has long been recognized as an important parameter of membrane bending elasticity by the biological physics community. Any molecular asymmetry between the two monolayers of a membrane creates curvature; as such this effect is important for morphological changes in the cell. Recently, there is a surge of activity in cell biology concerning spontaneous curvature. In which specific cellular mechanism is spontaneous curvature considered to be a key regulatory element? Do you know techniques to measure it?
And the answer is - endocytosis. Here spontaneous curvature is generated by initial protein binding to the membrane (later actin polymerization becomes important). Shape can be measured with electron tomography, see Kukulski et al., Plasma Membrane Reshaping during Endocytosis Is Revealed by Time-Resolved Electron Tomography, Cell Volume 150, Issue 3, 3 August 2012, Pages 508–520.
Interesting. Perhaps throw a fluorescence marker on a protein known to concentrate itself at regions of high curvature? Like alpha-synuclein?
Well, you need a (super-)resolution of 10 nm or better for this ... I think this is the main problem with any direct measurement of local spontaneous curvature: the length scales are tiny compared with 1. the typical size of vesicles / cell and 2. optical resolution.
Curvature mediated sorting of lipids/proteins can be accessible by experiments (e.g. sorting on tubes pulled out of a vesicles) through indirect means by measuring tether fluorescence intensity. These experiments though limited to biomimetic systems give a measure of the spontaneous curvature induced by proteins.
Few relevant experimental papers on this are :
1. Benoît Sorre et. al, Nature of curvature coupling of amphiphysin with membranes depends on its bound density. PNAS 2012 109 (1) 173-178;
2. Tian A, Baumgart T 2009. Sorting of lipids and proteins in membrane curvature gradients. Biophys J 96: 2676–2688
3. Roux et. al, Membrane curvature controls dynamin polymerization, PNAS 2010 107 (9) 4141-4146; doi:10.1073/pnas.0913734107
Another experimental technique can be small angle X-ray scattering (SAXS) to measure individual protein induced curvature contributions e.g. by Schmidt et. al, J. Am. Chem. Soc., 2013, 135 (37), pp 13710–13719 DOI: 10.1021/ja400146z
Apart from spontaneous curvature due to curvature inducing proteins, some recent works also claim there may be in addition a role of protein induced crowding itself thus independent of molecular shape of individual lipoproteins.
Two interesting papers on this aspect are:
1. Dannhauser, P. N. & Ungewickell, E. J. Nat. Cell Biol. 14, 634–639 (2012).
2. Stachowiak, J. et al. Nat. Cell Biol. 14, 944–949 (2012)
A direct/indirect measure of the role of spontaneous crvature in biological cells is elusive. In lieu of recent interests in ER morphology, it might be very interesting to find out using Super-resolution; the role of protein induced curvature in shaping these structures or if crowding plays a role.
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