Phosphate translocator

Phosphatase Hydrolysis of phosphate Translocation of ATPases linked to cation Inorganic pyrophosphatase (pyrophosphate orthophosphate + P043-) Glucose-6-phosphatase Fructose-1,6-bisphosphatase Alkaline phosphatase Phosphoprotein phosphatase (Na+, K+)-ATPase (Mg2+, Ca2+)-ATPase Proton-translocating ATPase Zn2+ Zn2+ Sodium pump Calcium pump H+ pump... 

To test the hypothesis that phosphate supply from the can limit the rate of photosynthesis (Sivak and Walker, 1986), antisense experiments were performed by Schultz et al. (1993). A cDNA for the potato triose phosphate translocator was identified and a fragment of this cDNA in reverse orientation was expressed in trangenic potato plants under the control of the constitutive cauliflower mosaic virus 35S promoter (Rismeier et al., 1993). This experiment confirmed that Pi supply can limit photosynthesis since a... 

Heldt, H. W. FlUgge, I., and Borchert, S. 1991. Diversity of specificity and function of phosphate translocators in various plastids. Plant Physiol. 95, 341-343. 

Rismeicr, J. W., Flugge, U. I., Schultz, B., Heinecke, D., Heldt, H. W., Wilmitzer, L., and Frommer, W. B. 1993. Antisense repression of of the chloropiast triose phosphate translocator affects carbon partitioning in transgenic potato plants. Proc. Natl. Acad. Sci. USA 90, 6160-6164. 

Schultz. B., Frommer. W. B., Flugge, U. I.. Hummel, S., Fischer, K., and Wilmitzer, L. 1993. Expression of the triose phosphate translocator gene from potato is light dependent and restricted to green tissues. Mol. Gen. Genet. 238, 357-361. 

The carrier protein facilitating Pj and phosphate ester transport is of particular interest in leaves in connection with carbon processing - i.e., the synthesis, transport and degradation of carbohydrate, all of which occur in the cytosol [51]. This metabolite carrier, called the phosphate translocator, is a polypeptide with a molecular mass of 29 kDa and is a major component of the inner envelope membrane [52,53]. The phosphate translocator mediates the counter-transport of 3-PGA, DHAP and Pj. The rate of Pj transport alone is three orders of magnitude lower than with simultaneous DHAP or 3-PGA counter-transport [54]. Consequently operation of the phosphate translocator keeps the total amount of esterified phosphate and Pj constant inside the chloroplast. Significantly, the carrier is specific for the divalent anion of phosphate. 

The principal physiological function of the phosphate translocator is to supply the cytosol with fixed carbon in the form of DHAP or 3-PGA. It has been dem-... 

The requirement of chloroplast photosynthesis for Pj and the release of Pj by sucrose synthesis in the cytosol require that these two processes be closely coordinated. Part of this coordination, as explained above, lies in the characteristics of the triose phosphate translocator. Results obtained in the last few years have led to the identification of a second component serving this function. Fructose 2,6-bis-phosphate (Fru-2,6-P2) coordinates the metabolism of sucrose, starch and CO2 fixation and, in so doing, links metabolic processes of the chloroplast with those of the cytosol. 

As discussed above, the Pj released in sucrose synthesis is recycled to the chlo-roplast, via the phosphate translocator in strict counter-exchange for triose phosphate [53]. 3-PGA can also be transported by this same carrier but, as also noted above, its export from the chloroplast in the light is restricted. 

In contrast, sucrose (common table sugar), a disaccharide, is synthesized in the cytosol. Plants lack the ability to transport hexose phosphates across the chloroplast membrane, but an abundant phosphate translocator mediates the transport of triose phosphates from chloroplasts to the cytosol in exchange for phosphate. Fructose 6-phosphate formed from triose phosphates joins the glucose unit of UDP-glucose to form sucrose 6-phosphate (Figure 20.13). Hydrolysis of the phosphate ester yields sucrose, a readily... 

Most studies with isolated hepatocytes or perfused liver indicate the existence of a ApH across the mitochondrial membrane, ranging from 0-0.6 [33,34,37-39], but under some conditions the calculated ApH depends on the metabolite chosen. This indicates disequilibrium in one or more of the transport steps. Under most conditions it is unlikely that the phosphate translocator is out of equilibrium because of its very high (Table 1). Problems do arise, however, when the dicarboxylate translocator is out of equilibrium because it connects the movement of citrate, malate and a-oxoglutarate with that of H. In that case the above equation cannot... 

In studies performed by Wohlrab [108] and by Hofmann and Kadenbach [109] a mitochondrial iimer membrane protein with an M of 31000-32000 with either Af-[ H]ethylmaleimide [108] or [ Hg]mersalyl [109] bound to it was isolated, which was claimed to be the phosphate translocator. Proof of its identity, however, can only be obtained if the protein is able to mediate phosphate transport in a reconstituted system. 

RATE-PHOSPHATE TRANSLOCATOR FROM SPINACH CHLOROPLASTS... 

As revealed by SDS-PAGE the translocator protein has a molecular weight of 29 kd and in its functional state it is built up of two identical subunits. It is coded for by nuclear genes and synthesized in the cytosol as a precursor protein (M 40 kd) (1). Recently we have succeeded in sequencing the cDNA encoding the entire spinach phosphate translocator precursor protein (2). Based on the deduced amino acid sequence a structural analysis of the translocator protein is presented. 

Predicted folding pattern of the chloroplast phosphate translocator in the inner envelope... 

Helical wheel projection of the transmembrane segments I,II,111,V and VI of the spinach chloroplast phosphate translocator. 

In wheat leaves, sucrose is the major storage carbohydrate and its concentration changes rapidly at the beginning of both the light and dark periods ClH. Sucrose synthesis occurs in the cytosol of mesophyll cells C2], from triose-phosphates generated within the chloroplast and exported by way of the phosphate translocator CSiJ. 

Structural Analysis of the Triose Phosphate-3-Phosphoglycerate-Phosphate Translocator... 

An external site of synthesis of PEP from photosynthetically formed 3-PGA can be detected by rising the P. concentration up to 5 mM and adding exogenously excessive amounts of rabbit PGM and enolase to intact chloroplasts (10 and 2.5 units/50 ug chlorophyll) similar as in /42/. The activity of the shikimate pathway (measured as nmol Phe and Tyr formed) was considerably enhanced as a result of supply of high amounts of PEP by this enzyme reaction and re-import by the phosphate translocator. Consequently, this way might be considered as an additional site of supply optimized under in vitro conditions by adding enzymes in excess. Under in vivo conditions, the carbon flow from triosephosphates of photosynthetic carbon fixation is preponderantly directed to sucrose synthesis /43/ and less to PEP. 

Heuer and Portis have recently synthesized a number of phosphonate compounds as potential inhibitors of the carboxylase reaction. Experiments with isolated chloroplasts showed that phosphonoformate and phos-phonopropionate entered the chloroplasts via the phosphate translocator in contrast to phosphonacetate, which did not. Furthermore, phosphonoformate was an effective inhibitor of 3-phosphoglycerate metabolism. As the authors noted, the amount of inhibitor that would have to be applied to whole plants would be considerable to ensure effective movement from cytoplasm to chloroplast stroma. ... 

Another class of potential inhibitors is the haloacetol phosphates of bromine, chlorine, and iodine. Hartman showed that there was irreversible binding of these compounds to the substrate active site of triose-phos-phate isomerase because of structural similarities to dihydroxyacetone phosphate.Subsequent work with isolated wheat chloroplasts has indicated that iodoacetol phosphates enter the chloroplast by the phosphate translocator and primarily inhibit glyceraldehyde-3-phosphate dihydro-... 

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