Mycoplasma fermentans seems to be involved in several pathogenic condtions in humans, and is among other things capable of fusing with T-cells and lymphocytes. The choline-containing phosphoglycolipid 6 H -O-(3 HH -phosphocholine-2 HH -amino-1 HH -phospho-1 HH ,3 HH -propanediol)-a-dglucopyranosyl-(1 H 33)-1,2-diacylglycerol (MfGL-II) in the membrane of M. fermentans has been suggested to enhance the fusion process, and the characteristics of MfGL-II were therefore investigated. When a cell culture ages the fraction of MfGL-II increases, and the fraction of the other major membrane lipid, phosphatidylglycerol (PtdGro), decreases concomitantly. Swelling experiments showed that the permeability and osmotic fragility are markedly reduced in aged cells. MfGL-II is selectively released into the surrounding medium when aged M. fermentans cells are incubated in buffer containing EDTA. The physico-chemical properties of MfGL-II were studied by NMR spectroscopy and differential scanning calorimetry, and they can explain the biochemical results. The temperature for the transition between gel and lamellar liquid crystalline (L a ) phases is 35±45 8C higher for MfGL-II than for PtdGro, which most probably gives rise to the reduced permeability in aged cells. At high water contents MfGL-II forms an L a phase and isotropic aggregates which were interpreted to be vesicles with a radius of < 450 A Ê . It is proposed that MfGL-II forms vesicles in the surrounding medium when it is released from the cell membrane. Neither EDTA nor Ca 21 ions have a significant influence on the aggregate structures formed by MfGL-II. Our results indicate that MfGL-II has no fusogenic properties. It is more probable that a recently identified lysolipid in the M. fermentans membrane acts as a fusogen.Keywords: Mycoplasma fermentans; MfGL-II; cholinecontaining lipids; membrane permeability; physico-chemical properties.The human pathogen Mycoplasma fermentans PG18 was first isolated from the urogenital tract several decades ago [1]. The interest in M. fermentans has recently increased because of reports indicating its possible role as a cofactor accelerating the progression of AIDS, its significance as a pathogen in other immunocompromised patients [2], and its role in the pathogenesis of rheumatoid arthritis [3]. While little is known of the molecular mechanisms underlying M. fermentans pathogenicity [4], it has been shown that HIV-associated cytopathic effects could be increased by the presence of M. fermentans [2], and that M. fermentans is capable of fusing with T-cells and peripheral lymphocytes [5].It is reasonable to assume that mycoplasmal membrane components are involved in the attachment and fusion of the microbe with eukaryotic host cells. Deutch et al. [6] isolated an unusual choline-containing phosphoglycolipid from the cell membranes of M. fermentans strain PG18 and suggested that this lipid was capable of enhancing the fusion of small, unilamellar vesicles with MOLT-3 lymphocytes in a dose-dependent manner. Structural analysis of this l...
The molecular orientation in a lipid membrane of the peptide fragment VEYAGIALFFVAAVLTLWSMLQYLSAAR (phosphatidylglycerophosphate synthase (Pgs) peptide E) of an integral membrane protein, Pgs, in Escherichia coli has been investigated by solid-state 15N nuclear magnetic resonance (NMR) on macroscopically aligned lipid bilayers. The secondary structure of the peptide in lipid vesicles was determined by circular dichroism spectroscopy. Furthermore, the phase behaviour of the Pgs peptide E/dierucoylphosphatidylcholine (DEruPC)/water system was determined by (2)H, (31)P and 15N solid-state NMR spectroscopy. The phase behaviour obtained was then compared to that of the Pgs peptide E solubilised in dioleoylphosphatidylcholine and water that was previously studied by Morein et al. [Biophys. J. 73 (1997) 3078-3088]. This was aimed to answer the question whether a difference in the length of the hydrophobic part of this peptide and the hydrophobic thickness of the lipid bilayer (hydrophobic mismatch) will affect the phase behaviour. The peptide mostly has a transmembrane orientation and is in an alpha-helical conformation. An isotropic phase is formed in DEruPC with high peptide content (peptide/lipid molar ratio (p/l) > or =1:15) and high water content (> or =50%, w/w) at 35 degrees C. At 55 and 65 degrees C an isotropic phase is induced at high water content (> or =50%, w/w) at all peptide contents studied (no isotropic phase forms in the lipid/water system under the conditions in this study). At high peptide contents (p/l> or =1:15) an isotropic phase forms at 20 and 40% (w/w) of water at 55 and 65 degrees C. A comparison of the phase behaviour of the two homologous lipid systems reveals striking similarities, although the thicknesses of the two lipid bilayers differ by 7 A. This suggests that the rationalisation of the phase behaviour in terms of the hydrophobic mismatch is not applicable to these systems. The C-terminus of Pgs peptide E is amphiphilic and a considerable part of the peptide is situated outside the hydrophobic part of the bilayer, a property of the peptide that to a large extent will affect the lipid/peptide phase behaviour.
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