Labeling of membranes and reaction centers from the photosynthetic bacterium rhodospirillum rubrum with fluorescamine

2-[8-14C]naphthyl 2-diazo-3,3,3-trifluoropropionate, a new carbene generating reagent for probing hydrophobic membrane domains

A new precursor of a lipophilic photolabel, 2-[8-14C]naphthyl 2-diazo-3,3,3-trifluoropropionate (NADIT) has been synthesized. The suitability of the reagent for labeling the hydrophobic core of membranes is demonstrated by studying its reactivity in chromatophores of Rhodospirillum rubrum G-9+. The label binds preferentially to the phospholipids and intrinsic membrane proteins. In isolated reaction centers treated with NADIT the hydrophobic subunits M and L are more labeled than the H subunit. The high reactivity, dark stability and ease of synthesis favors this very lipophilic reagent to identify the intrinsic hydrophobic sections of membrane proteins.

Effects of energization on membrane organization in mycoplasma

Fluorescence polarization and ESR experiments using various probes demonstrated that addition of glucose to resting Mycoplasma capricolum and Mycoplasma mycoides subs capri had, if any, a very limited effect on the physical state of their membrane lipids. Under the same conditions the degree of exposure of primary amino groups of membrane proteins to the aqueous surrounding, estimated from fluorescence labeling by fluorescamine and the cycloheptaamylose-fluorescamine complex was significantly increased. This energy dependent increase was blocked by dicyclohexylcarbodiimide (DCCD), an inhibitor of the membrane bound Mg2+ stimulated ATPase of mycoplasma and by carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) which, in mycoplasma, only affects the chemical component of the proton-motive force. Variations in the proton activity gradient across the membrane induced by changing the pH of the labeling medium resulted in parallel variations in the ratio of relative intensities of labeling of energized to resting cells. The values taken by this ratio were up to two for a maximal proton gradient of 0.9 pH unit and tended to unity when the intracellular and extracellular pH tended to equalize. It is concluded that, upon mycoplasma cell energization, membrane proteins undergo a conformational change resulting in the exposure of new free amino groups. This conformational change is primarily dependent on the existence of a delta ph across the membrane and occurs in the absence of important modifications in the physical state of membrane lipids.

Lateral organization of proteins in the chromatophore membrane of Rhodospirillum rubrum studied by chemical cross-linking

The organization of proteins in the chromatophore membrane, particularly of the reaction center and the light-harvesting polypeptide, was examined by the use of a hydrophobic and a hydrophilic cross-linking reagent, namely DSP (dithiobis-succinimidyl propionate) and glutaraldehyde. The linkage of proteins was studied by SDS polyacrylamide pore gradient electrophoresis. DSP was shown to link proteins within the core of the membrane. The subunit H of the reaction center is linked with DSP at a low concentration, either with itself or with other membrane proteins but not to the subunits M and L. In isolated reaction centers the subunits H are exclusively linked with each other. With increasing concentrations of DSP the bands of the subunits M, L, and the light-harvesting polypeptide disappear simultaneously from the gel, suggesting that these proteins are linked together. This hypothesis is supported by the finding that reaction centers isolated from chromatophores treated with DSP retain an appreciable amount of light-harvesting polypeptide. With increasing concentrations of the hydrophilic cross-linking reagent glutaraldehyde, the bands of all the three subunits of the reaction center, H, M, and L, progressively disappear from the gel, suggesting that they are linked together. The light-harvesting polypeptide remains free when this reagent is used.