title: Facilitated water transport in cyanobacterium Synechococcus sp. PCC 7942 studied by phycobilisome-sensitized chlorophyll a fluorescence.
abstract: Water transport across plant cell membranes is difficult to measure. We present here a model assay, based on chlorophyll (Chl) a fluorometry, with which net water transport across the cell membrane of freshwater cyanobacterium Synechococcus sp. PCC7942 (S7942) can be followed kinetically with millisecond-time resolution. In cyanobacteria, the phycobilisome (PBS)-sensitized Chl a fluorescence increases when cells expand (e.g., in hypo-osmotic suspension) and decreases when cells contract (e.g., in hyper-osmotic suspension). The osmotically-induced Chl a fluorescence changes are proportional to the reciprocal of the suspension osmolality (DeltaF proportional, variant Osm-1; Papageorgiou GC and Alygizaki-Zorba A (1997) Biochim Biophys Acta 1335: 1-4). In our model assay, S7942 cells were loaded with NaCl (passively penetrating solute) and shrunk in hyper-osmotic glycine betaine (nonpenetrating solute). Upon injecting these cells into hypo-osmotic medium, the PBS-sensitized Chl a fluorescence rose to a maximum due to the osmotically-driven water uptake. The rise of Chl a fluorescence (water uptake) was partially inhibited by HgCl2, at micromolar concentrations. Arrhenius plots of the water uptake rates gave activation energies of EA=4.9 kcal mol-1, in the absence of HgCl2, and EA=11.9 kcal mol-1 in its presence. These results satisfy the usual criteria for facilitated water transport through protein water pores of plasma membranes (aquaporins), namely sensitivity to Hg2+ ions and low activation energy.
authors.affiliation: lnstitute of Biology, National Center for Scientific Research Demokritos, Athens 15310, Greece.
authors.forename: Kostas
authors.initials: K
authors.lastname: Stamatakis
authors.forename: Ladas
authors.initials: L
authors.lastname: Nectarios
authors.forename: George C
authors.initials: GC
authors.lastname: Papageorgiou
mesh_terms: D001692:Biological Transport; D002734:Chlorophyll / Q000737:chemistry / Q000378:metabolism*; D000077194:Chlorophyll A; D005453:Fluorescence; D009994:Osmolar Concentration; D045524:Phycobilisomes / Q000378:metabolism*; D046940:Synechococcus / Q000737:chemistry / Q000378:metabolism*; D013997:Time Factors; D014867:Water / Q000378:metabolism*
chemical_list: D045524:Phycobilisomes; D014867:Water; D002734:Chlorophyll; D000077194:Chlorophyll A