Na+-dependent K+ uptake Ktr system from the cyanobacterium Synechocystis sp. PCC 6803 and its role in the early phases of cell adaptation to hyperosmotic shock.
Transmembrane ion transport processes play a key role in the adaptation of cells to hyperosmotic conditions. Previous work has shown that the disruption of a ktrB/ntpJ-like putative Na(+)/K(+) transporter gene in the cyanobacterium Synechocystis sp. PCC 6803 confers increased Na(+) sensitivity, and inhibits HCO(3)(-) uptake. Here, we report on the mechanistic basis of this effect. Heterologous expression experiments in Escherichia coli show that three Synechocystis genes are required for K(+) transport activity. They encode an NAD(+)-binding peripheral membrane protein (ktrA; sll0493), an integral membrane protein, belonging to a superfamily of K(+) transporters (ktrB; formerly ntpJ; slr1509), and a novel type of ktr gene product, not previously found in Ktr systems (ktrE; slr1508). In E. coli, Synechocystis KtrABE-mediated K(+) uptake occurred with a moderately high affinity (K(m) of about 60 microm), and depended on both Na(+) and a high membrane potential, but not on ATP. KtrABE neither mediated Na(+) uptake nor Na(+) efflux. In Synechocystis sp. PCC 6803, KtrB-mediated K(+) uptake required Na(+) and was inhibited by protonophore. A Delta ktrB strain was sensitive to long term hyperosmotic stress elicited by either NaCl or sorbitol. Hyperosmotic shock led initially to loss of net K(+) from the cells. The Delta ktrB cells shocked with sorbitol failed to reaccumulate K(+) up to its original level. These data indicate that in strain PCC 6803 K(+) uptake via KtrABE plays a crucial role in the early phase of cell turgor regulation after hyperosmotic shock.
Bioscience and Biotechnology Center, Nagoya University, Nagoya 464-8601, Japan.
Nobuyuki
N
Matsuda
Hiroshi
H
Kobayashi
Hirokazu
H
Katoh
Teruo
T
Ogawa
Lui
L
Futatsugi
Tatsunosuke
T
Nakamura
Evert P
EP
Bakker
Nobuyuki
N
Uozumi
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D027682:Cation Transport Proteins / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D004926:Escherichia coli / Q000235:genetics; D015870:Gene Expression; D006863:Hydrogen-Ion Concentration; D008564:Membrane Potentials / Q000502:physiology; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D008969:Molecular Sequence Data; D009994:Osmolar Concentration; D009997:Osmotic Pressure; D011188:Potassium / Q000378:metabolism*; D011994:Recombinant Proteins; D012964:Sodium / Q000494:pharmacology; D046939:Synechocystis / Q000737:chemistry* / Q000235:genetics; D014882:Water-Electrolyte Balance
D001426:Bacterial Proteins; D027682:Cation Transport Proteins; C113206:KtrB protein, Bacteria; D008565:Membrane Proteins; D011994:Recombinant Proteins; D000255:Adenosine Triphosphate; D012964:Sodium; D011188:Potassium