Virus-encoded movement protein (MP) mediates cellto-cell spread of tobacco mosaic virus (TMV) through plant intercellular connections, the plasmodesmata. The molecular pathway by which TMV MP interacts with the host cell is largely unknown. To understand this process better, a cell wall-associated protein that specifically binds the viral MP was purified from tobacco leaf cell walls and identified as pectin methylesterase (PME). In addition to TMV MP, PME is recognized by MPs of turnip vein clearing virus (TVCV) and cauliflower mosaic virus (CaMV). The use of amino acid deletion mutants of TMV MP showed that its domain was necessary and sufficient for association with PME. Deletion of the PME-binding region resulted in inactivation of TMV cell-to-cell movement.

Ion movements associated with the pH rise that is observed upon illumination of thylakoid suspensions at low pH have been studied by a multiparameter technique. Light-dependent, dark-reversible fluxes of H+, Cl-, Na +, K + and divalent cations were monitored, together with simultaneous changes in the optical density of the suspension. Extensive uptake of Cl-and efflux of Mg2 + accompany the apparent inward movement of H + in the light. Only minor efflux of K+ is seen and Na+ appears immobile. The Cl-and Mg2+ fluxes together compensate for most of the charge transferred as H+, contributing respectively about 49% and 43% on an equivalent basis. The ratio of Cl-influx to Mg2 + efflux is variable, but usually >1.0., The Mg2 + flux can be supplanted by (1) K +flux, if the K+/Mg2+ activity ratio in the suspension is high, and (2) Ca2+ flux, if the thylakoids are equilibrated with suspending media containing Ca2+. The affinity of the divalentcation-binding sites, or carrier mechanism, is greater for Ca2 + than for Mg2 +. Schemes can be drawn up to account for the observed ion movements on the basis of either a chemical or a chemiosmotic mechanism for energy transduction in chloroplasts. In intact chloroplasts, lightdependent control of Mg2 + distribution between thylakoid and stroma could serve to regulate enzyme activities in the carbon fixation pathway, and hence photosynthesis.Ten years ago, the first description appeared (1) of the lightdriven apparent uptake of hydrogen ions by isolated thylakoids (chloroplast inner membranes). The pH rise in the suspending medium was associated with a reversible increase in sample turbidity, and with the ability of energized thylakoids to phosphorylate ADP, upon relaxation in the dark under conditions conducive to photophosphorylation.These observations have been extensively elaborated, and reviewed in detail (2, 3). The pH rise phenomenon attracts continued attention, since it has seemed to provide the most tangible evidence in favor of the chemiosmotic mechanism (4) of energy transduction. Estimates of up to 3.0 pH units have been made (5-7) for the magnitude of the H+ gradient across illuminated thylakoid membranes, and this has been construed as establishing the thermodynamic feasibility of chemiosmotic coupling.Such massive pH shifts cannot occur without concomitant flux of some ion other than H+, as required by the law of electroneutrality. It is commonly assumed that Cl-serves as a counterion, and Cl-uptake has been observed (8-10). That reported by Deamer and Packer (10) was seen under conditions favoring uptake of the phenazine methosulfate cation (11), in a manner presumably analogous to the uptake of amine cations and Cl-(12). A simultaneous demonstration of stoichiometric H+ and Cl-uptake in the absence of uncoupler has yet to be given. Indeed, Dilley and Vernon (13) Abbreviations: HEPES, N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid; PIPES, piperazine-N,N'-bis(2-ethanesulfonic acid).

Circular dichroism (CD) and magnetic circular dichroism (MCD) spectra were recorded for spinach thylakoids and for isolated, aggregated chlorophyll a / b light-harvesting pigment-protein complex, in random and magnetically aligned states of orientation at room and low temperatures. The shape and magnitude of the C D signal of most bands strongly depended on the orientation of the thylakoid membranes or the aggregated pigment-protein complex. In both thylakoids and aggregated light-harvesting complexes, however, the MCD spectra of the two different orientations were almost identical. Random and magnetically aligned samples exhibited anomalous, large C D signals outside the bands of pigment absorbance. Lack of similarity between the corresponding M C D and C D spectra showed that the large C D signals are not produced as a distortion of CD of absorbance by light scattering. Instead, these anomalous spectral features are believed to originate in differential selective scattering of circularly polarized light. Our results lead to the conclusion that the light-harvesting pigment-protein complex in thylakoid grana forms a helical macroarrav with dimensions commensurate with the wavelengths of the anomalous circular dichroism signals. A hypothesis is put forward suggesting a role for C i r c u l a r dichroism (CD)' is a powerful technique, yielding structural information on biological systems. CD of absorbance (CDA), an intrinsic property of chiral molecules, may also arise from short-or long-range coupling between chromophores of

The organization of pigment-protein complexes into large chiral macrodomains was investigated in wild-type and chlorophyll b-less mutant thylakoid membranes of barley. The variations in the anomalous circular dichroism bands and in the angular-dependence of circular intensity differential scattering showed that in wild-type chloroplasts, the formation of macrodomains was governed by interactions of the light-harvesting chlorophyll alb complexes (LHCII). Two external factors could be identified which regulate the parameters of the anomalous circular dichroism signal: (i) electrostatic screening by divalent cations under conditions that favor membrane stacking and (ii) the osmotic pressure of the medium, which is suggested to affect the lateral interactions between complexes and influence the packing-density of particles. These two factors governed preferentially the negative and the positive anomalous circular dichroism signals, respectively. In the chlorina f-2 mutant thylakoid membranes, deficient in most chlorophyll b binding proteins, the formation of macrodomains which gave rise to the anomalous circular dichroism signals was still regulated by these same external factors. However, in the absence of major LHCII polypeptides the formation of macrodomains was apparently mediated by other complexes having weaker interaction capabilities. As a consequence, the size of the macrodomains under comparable conditions appeared smaller in the mutant than in the wild-type thylakoid membranes.Circular dichroism is a valuable probe for examining the long-range interactions between pigment-protein complexes which participate in the formation and stabilization of membrane ultrastructure. A functional role of macrodomains in long-range energy migration processes is proposed. 273

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