31P NMR studies of vegetative and encysted cells of Acanthamoeba castellanii. Observation of phosphonic acids in live cells

Screening for carbon-bound phosphorus in marine animals by high-resolution 31P-NMR spectroscopy: coastal and hydrothermal vent invertebrates

Animals of hydrothermal vents live in a unique environment that conceivably could lead to modifications of the usual phosphorus functional groups of importance in living systems. To explore this possibility, specimens of a sea anemone (unidentified) from the TAG hydrothermal field, Mid-Atlantic Ridge, the mussel Bathymodiolus N. sp. from the Mid-Atlantic Ridge, and the tubeworm Riftia pachyptila from the East Pacific Rise were analyzed for compounds containing the carbon&z.sbnd;phosphorus bond. The analysis was based on the use of 31P-nuclear magnetic resonance, which gives signals for C-P compounds that are well separated from those of biological phosphoric acid derivatives. The animals were extracted to provide a lipid- and a water-soluble fraction, leaving an insoluble, largely proteinaceous solid residue. The lipid and residue fractions were subjected to hydrolysis to release bound forms of phosphonic acids. All fractions were analyzed by 31P-NMR. Aminophosphonic acids [primarily NH2CH2CH2PO(OH)2 (1) and CH3NHCH2CH2PO(OH)2 (2)] represented the only type of C-P compound detected. These are well-known constituents of coastal invertebrates. For the mussel and sea anemone, these compounds were present in bound form in both the lipid and insoluble residue. The tube worm contained C-P material only in the insoluble residue, but in quite small amounts. The 31P-NMR method is especially valuable in being able to discriminate between compounds 1 and 2. By this technique, two coastal sea anemones (Tealia felina and Bunadosoma cavernata), previously thought to have 1 as the dominant aminophosphonic acid, were in fact found to be much richer in originally undetected 2. This compound was also detected for the first time in a mussel (Genkensia demissa).

Contraction of reconstituted Dictyostelium cytoskeletons: an apparent role for higher order associations among myosin filaments

A large number of cellular functions require assembly of actin and myosin and coordinated interactions between the resulting filaments. To better understand the structure and function of one such contractile assembly, we have begun fractionation and reconstitution studies of Dictyostelium cytoskeletons. Isolated cytoskeletons rapidly contracted when mixed with Mg-ATP, and myosin II was essential for this since myosin-depleted (stripped) cytoskeletons failed to contract. Dictyostelium, Acanthamoeba, or skeletal muscle myosins bound to stripped cytoskeletons with equal efficiency, and the Mg-ATPase of all three myosins was stimulated by the cytoskeleton-associated actin. Near neutral pH, however, only the homologous system reconstituted with Dictyostelium myosin contracted, despite the fact that under the same conditions all three myosins bound to myosin-depleted (ghost) muscle myofibrils and restored contractility. Individual Dictyostelium myosin thick filaments have a strong tendency to aggregate and associate end-to-end, and this may be important for functional contraction of cytoskeletons. This suggestion is supported by the observation that under conditions where individual Acanthamoeba myosin filaments aggregated, reconstituted cytoskeletons contracted. None of the solution conditions tested caused rabbit muscle myosin filaments to aggregate or to contract cytoskeletons. Thus higher order associations among individual myosin filaments may be essential for some types of cell motility.

Kinetics of endosomal acidification in Dictyostelium discoideum amoebae. 31P-NMR evidence for a very acidic early endosomal compartment

We have examined the pH of the various endosomal compartments in the amoebae of the cellular slime mould Dictyostelium discoideum. This was accomplished both by fluorescence and by in vivo 31P-NMR methods. The fluid-phase marker, fluorescein-labeled dextran, was fed to the amoebae to report the average pH of their endocytic vesicles. During the progressive loading of successive endosomal compartments, we observed an early acidification down to a minimum value of pH < or = 5.3 after 30 min at 20 degrees C followed by an increase to an average pH of 5.8 when all the endosomal compartments were loaded by the fluid-phase marker. The weak fluorescence intensity of FITC-dextran at acidic pH precluded a more detailed investigation and we checked various phosphonate compounds as potential 31P-NMR pH probes for the endosomal compartments. Two molecules, aminomethylphosphonate and 2-aminoethylphosphonate, were selected for this study because of the large amplitudes of their chemical shift variation with pH (2 and 2.5 ppm, respectively) and their acidic pKs of 5.5 and 6.3, respectively. They were only moderately toxic (IC50% approximately 10 mM) towards both the axenic growth and the differentiation program of Dictyostelium amoebae. Internalization of the two aminophosphonates occurred only through the fluid-phase pinocytosis pathway as revealed by the full inhibition of their entry with 1 mM vanadate or 7.5 mM caffeine, two previously characterized inhibitors of endocytosis in Dictyostelium. We found that in vivo 31P-NMR of amoebae suspensions incubated with the aminophosphonates allowed the detection of three distinct intracellular compartments at pH 4.3, 5.8-6.0 and 7.3. Kinetics of aminophosphonate entry were analyzed and the results allowed us to reconstruct the time course for the acidification sequence during endocytosis. The data are consistent with the hypothesis that in Dictyostelium amoebae phosphonates occupy a highly acidic early endosomal compartment (t1/2 = 18 min; pH 4.3) before reaching a less acidic late endosomal/prelysosomal compartment (pH 5.8-6.0) from where they are immediately transported to, and trapped in, the cytoplasm (pH 7.3).

Identification of a naturally occurring methyl-ester of phosphate, methyl-phosphorylcholine (methyl-2-(N,N,N trimethylamino) ethyl phosphate), in the eggs of the sea urchin S. purpuratus

A novel metabolite of choline, phosphorylcholine methyl ester, has been identified in the eggs of S. purpuratus wherein it is present at approximately 1 mM concentration. To the best of our knowledge, this is the first instance of a phosphoryl-methyl-ester to be observed in nature. The compound appears to be species specific, since it has not been observed in other species such as L. pictus and P. depressus. In S. purpuratus its distribution is confined to the ovary, eggs and embryos, and is absent from young animals following metamorphosis.

The effect of heavy chain phosphorylation and solution conditions on the assembly of Acanthamoeba myosin-II

At low ionic strength, Acanthamoeba myosin-II polymerizes into bipolar minifilaments, consisting of eight molecules, that scatter about three times as much light as monomers. With this light scattering assay, we show that the critical concentration for assembly in 50-mM KCl is less than 5 nM. Phosphorylation of the myosin heavy chain over the range of 0.7 to 3.7 P per molecule has no effect on its KCl dependent assembly properties: the structure of the filaments, the extent of assembly, and the critical concentration for assembly are the same. Sucrose at a concentration above a few percent inhibits polymerization. Millimolar concentrations of MgCl2 induce the lateral aggregation of fully formed minifilaments into thick filaments. Compared with dephosphorylated minifilaments, minifilaments of phosphorylated myosin have a lower tendency to aggregate laterally and require higher concentrations of MgCl2 for maximal light scattering. Acidic pH also induces lateral aggregation, whereas basic pH leads to depolymerization of the myosin-II minifilaments. Under polymerizing conditions, millimolar concentrations of ATP only slightly decrease the light scattering of either phosphorylated or dephosphorylated myosin-II. Barring further modulation of assembly by unknown proteins, both phosphorylated and dephosphorylated myosin-II are expected to be in the form of minifilaments under the ionic conditions existing within Acanthamoeba.

Microinjection into Acanthamoeba castellanii of monoclonal antibodies to myosin-II slows but does not stop cell locomotion

To study the in vivo role of myosin-II in Acanthamoeba castellanii, motile cells were microinjected with monoclonal antibodies raised against the myosin-II heavy chain. All injected cells underwent a transient shock response. It was found that although injection of buffer alone or of an endogenous Acanthamoeba protein decreased the motility of injected cells from 7 microns/min to approximately 3 microns/min, injection of monoclonal antibodies specific for myosin-II decreased motility further to approximately 0.8 micron/min. This effect was seen whether or not the monoclonal antibody to myosin-II inhibited the actomyosin-II MgATPase activity in vitro. Levels of antibody far in excess of endogenous myosin-II concentrations could not completely block amoeboid movement. The morphology of moving antimyosin-II-injected cells was unusual, suggesting a greater defect in the ability to retract the trailing edge of the cell rather than to extend the leading edge. Endosomes frequently disappeared from injected cells, and although buffer-injected cells rapidly recovered visible endosomes (50% recovery at 5 min), endosomes were not seen in antimyosin-II-injected cells until, on the average, approximately 50 min after injection. Injection of a nonspecific antibody or of a nonspecific exogenous protein (ovalbumin) also decreased the mobility of the injected cells beyond that of buffer-injected cells (to approximately 1 micron/min). These cells tended to recover endosomes more rapidly (approximately 25 min) than cells injected with antimyosin-II monoclonal antibodies. The inability of antibodies to myosin-II to inhibit completely any of the movements studied suggests that although myosin-II probably plays a role in these motilities, the cell either routinely uses or can draw upon another cytoplasmic motor to maintain locomotion, organelle movement, contractile vacuole activity, and endocytosis.

Prospects for NMR imaging in the study of biological morphogenesis

Small objects can be visualised with a spatial resolution that approaches microscopic dimensions using the technique of high resolution nuclear magnetic resonance (NMR) imaging. Some important features of the method are described and the prospects for using the technique to study morphogenesis are discussed. It is concluded that NMR imaging, in conjunction with the related method of localised spectroscopy, is capable of producing novel structural information.

A versatile perfusion technique for metabolic studies by NMR

A perfusion technique applicable to metabolic studies by NMR is described in detail. The utility and versatility of the approach are demonstrated by following gluconeogenesis from [2-13C]pyruvate in the perfused, isolated mouse liver, and lipogenesis from [2-13C]acetate in perfused Acanthamoeba castellanii cells embedded in agarose filaments.

A 31P-NMR study of structural and functional aspects of phosphate and phosphonate distribution in Tetrahymena

31P-NMR has been used to study the chemical nature of cytoplasmic components of live Tetrahymena in a non-invasive manner. The technique has further been used to characterized the physical behaviour of lipids extracted from this organism. In particular, we have shown the presence of large quantities of pyrophosphate and of tripolyphosphate in acid extracts of the organism. These are not detectable in the live cells due to the motionally rigid nature of the storage granules. We have characterized the distribution of phosphonic acids in the organism and followed the phase behavior of the extracted cell lipids. Aqueous dispersions of extracted lipid show both bilayer and non-bilayer behaviour in the range of the growth temperature. The phosphonolipid in Tetrahymena appears to play a role similar to that of phosphatidylethanolamine in regulating the phase behaviour of the membrane. The high degree of unsaturation in the fatty acids of Tetrahymena is most likely responsible for the polymorphic phase behaviour observed near the growth temperature.