It seems to me that the spontaneous curvature of most lipids are well determined, for example, as listed in this wikipedia page:
http://en.wikipedia.org/wiki/Membrane_curvature (Lipid Spontaneous Curvature section).
However, after tedious literature search, I was surprised that no reported value about the spontaneous curvature of phosphatidylinositol, especially PI4,5P2, can be found. If you are aware of papers talking about this issue, or happen to have measured PIP2 curvature yourself, could you let me know?
Dear Zheng,
I saw your query and would like to add- In my understanding the PIP are very small structures which may cluster and form locally some self assembled structures which can then induce a very "different" local curvature on the membrane. Hence this is difficult to say what is the "intrinsic" curvature of a individual PIP. Also this idea of a intrinsic spontaneous curvature, is probably of less significance, if we agree that these proteins have membrane mediated interactions which can cause much different induced curvatures depending on the concentration of the proteins and their mutual interactions on the membrane. Such concentration dependent measures of spontaneous curvature has been recently reported in literature for ex: for the proteins in bar family.
What is new and exciting is, is that apart from intrinsic curvatures; in a experimental system another interesting contribution comes from crowding effects. Due simple macromolecular crowding you can bend membranes and thus measured values can have hidden contributions due to crowding effects which until recent years had been unknown. This aspect has been discussed in the review article by T. Kirchaussen : http://www.nature.com/ncb/journal/v14/n9/full/ncb2570.html and also in this more recent article L. Johannes : http://cshperspectives.cshlp.org/content/6/1/a016741.abstract
I guess, considering these aspects, for single protein curvature measurements are of much interests and to my knowledge there is exciting work from G.C.L. Wong"s group (Measure of Gaussian curvature of Influenza M2 Protein: http://pubs.acs.org/doi/abs/10.1021/ja400146z , Curvature measurements of Antimicrobial peptides : http://www.sciencedirect.com/science/article/pii/S1359028613000752 ) .
Best,
Sabya
Hi Sabya,
Thanks for your reply and nice explaination. But I think I'm looking for a more physical view of the lipid bilayer. I understand phosphatidylinositols are essential lipids mostly because they can interact with different peripheral proteins, and spontaneous curvature arised from protein-lipid interactions can be very complicated as you said.
But if we only look at the lipid bilayer, I think the shape of inidvidual lipids can also be interesting, at least biophysically. For example, the shape or spontaneous curvature of lipids will determine the prefered curvature of the monolayer. When two monolayers form a bilayer, the pressure profile should be somewhat related to the original shape of the monolayers. And the pressure profile across a lipid bilayer could also influence the insertion properties of proteins. The spontaneous curvature of different lipids in the wikipeida page such as for DOPE and DOPC, are mostly measured by x-ray diffraction from lipid hexagonal phases. So it should be a pure physical parameter of this lipid species, but again I agree with you about how protein-lipid interactions can make this much more complicated and also more meaningful.
Best,
Zheng
Dear Zheng,
Physically, the "spontaneous curvature" of a lipid is a quite ill-defined notion. If you could make a bilayer membrane only out of this lipid, its spontaneous curvature would be zero (because the two layers are identical). Therefore, the spontaneous curvature of lipid is only defined as a comparison to another lipid (as it is enriched in one leaflet) - as it is written in the wikipedia article you quote.
As mentioned above, many things will change the effect of a lipid on the membrane (ex: proteins in or at the membrane), and the "spontaneous" curvature of a pure lipid (if it was correctly defined) would most likely not be a good description of the effect of this lipid in a highly complex environment.
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