During my master degree I had the opportunity to work during 9 months with the Pr. Henri Wróblewski at the University of Rennes 1 (UMR6026-Molecular and Cellular Interactions, group NMR and lipids-proteins interactions).
The group was interested in the study of Mollicutes, wall-less bacteria that are pathogenic for mammals, plants, and insects. Several characteristics make Mollicutes a very powerful tool for the study of membrane properties. They lack a cell-wall. So, it is theoretically possible to alter the lipid composition by simply adding drugs to the medium. Moreover, some of them are auxotrophic for most of the lipids. The membrane composition of the organism is therefore directly influenced by the kind of lipids you feed them with. On the contrary to other bacteria, they have sterols and sphingomyelin in their membrane, making them a good model for the study of euckaryotic-like membranes.
Spiroplasma is a genus of Mollicutes. These bacteria have a helical shape and move in a corkscrew motion. This kind of motion is thought to favor the movement in viscous fluids such as the phloem of plants. Figure from Cole et al., 1973, J. Bacteriol., 115:367.
Spiralin is an acyl-lipoprotein that is anchored in the outer leaflet of the plasma membrane of Spirolpasma. It represents 25% in mass of the plasma membrane proteins! Considering that spiralin is located only in the outler leaflet of the membrane, one can estimate that half of the membrane proteins facing the medium is spiralin. There is only poor information about the structure of spiralin. Previous studies reported on the acylation and lipoylation of spiralin and on its secondary structures using circular dichroism, FT-IR spectroscopy, NMR and computational approaches. It appeared that spiralin is a globular alpha/beta protein having 25-28% of alpha helices and 32-38% of beta-sheets. Spiralin is tensio-active but the peptide part of the protein interacts only with the polar head groups of the membrane.
The function of spiralin remains elusive. Nevertheless, it has been shown that spiralin from Spiroplasma citri acts as a lectin in vitro and binds glycoproteins from its insect vector Circulifer haematoceps (Killiny et al., 2005). Moreover, it has been proposed that spiralin is required only for the efficient transmission of S. citri by its insect vector (Duret et al., 2003).
Spiralin is also a macrophage-stimulating lipopeptide and this activity resides in the N-terminal part of the protein. Given the potent immunostimulating activity of spiralin, the first training that I did in the laboratory was to purify spiralin and try to find the best conditions to produce immnuno-stimulating complexes (ISCOMs) containing spiralin.
During the second training that I did in this laboratory, I tried to identify potential interactions between spiralin and plasma membrane lipids. Spiralin is highly concentrated in the outer leaflet of the plasma membrane which contains high amounts of cholesterol and sphingomyelin. These two lipids are known to be enriched in eukaryotic membranes in the so-called lipid rafts. Is it possible that Spiroplasma membranes contain such lipid domains? Where would spiralin be distributed? Could this domains have an importance for the helical shape of these bacteria?