Table 2.

Determination of Substrate Specificities of Recombinant Phosphate Translocators from Heterotrophic Tissues and Chloroplasts Expressed in Yeast Cells

Recombinant (Histidine-Tagged) Translocator Proteins Purified from S. pombe Cellsb
Internal Liposomal SubstrateaGPT Pea Root PlastidsPPT Cauliflower Bud PlastidscTPT Spinach Chloroplastsc
Phosphate= 100= 100= 100
TrioseP112 ± 522 ± 392 ± 17
3-PGA50 ± 116 ± 390 ± 16
PEP20 ± 172 ± 95 ± 2
Glc6P90 ± 92 ± 15 ± 4
Glc1P<14 ± 5NDd
  • a The liposomes had been preloaded with 25 mM substrates as indicated.

  • b The transport activities of the pea root GPT–His6, the cauliflower PPT–His6, and the spinach TPT–His6 proteins, respectively, that had been purified to apparent homogeneity by Ni2+–nitrilotriacetic acid chromatography from S. pombe cells were reconstituted into the liposomes. Transport of 32P-phosphate was measured as described in Methods and is given as a percentage of the activity measured for proteoliposomes preloaded with inorganic phosphate (minus values obtained for liposomes containing only buffer). The 100% exchange activities (micromoles per milligram of protein per minute) of the recombinant proteins were 1.2 (GPT), 1.5 (PPT), and 0.85 (TPT), respectively. Mean values are from three to five different experiments ±SE.

  • c Data from Fischer et al. (1997).

  • d ND, not determined.