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Identification and Regulation of Plasma Membrane Sulfate Transporters in Chlamydomonas

Pootakham, Wirulda, Gonzalez-Ballester, David, Grossman, Arthur R.
Plant physiology 2010 v.153 no.4 pp. 1653-1668
Chlamydomonas reinhardtii, acclimation, anions, dose response, genetics, mutants, plasma membrane, proteasome endopeptidase complex, starvation, sulfates, transporters
Chlamydomonas (Chlamydomonas reinhardtii) exhibits several responses following exposure to sulfur (S)-deprivation conditions, including an increased efficiency of import and assimilation of the sulfate anion (SO₄²⁻). Aspects of SO₄²⁻ transport during S-replete and S-depleted conditions were previously studied, although the transporters had not been functionally identified. We employed a reverse genetics approach to identify putative SO₄²⁻ transporters, examine their regulation, establish their biogenesis and subcellular locations, and explore their functionality. Upon S starvation of wild-type Chlamydomonas cells, the accumulation of transcripts encoding the putative SO₄²⁻ transporters SLT1 (for SAC1-like transporter 1), SLT2, and SULTR2 markedly increased, suggesting that these proteins function in high-affinity SO₄²⁻ transport. The Chlamydomonas sac1 and snrk2.1 mutants (defective for acclimation to S deprivation) exhibited much less of an increase in the levels of SLT1, SLT2, and SULTR2 transcripts and their encoded proteins in response to S deprivation compared with wild-type cells. All three transporters were localized to the plasma membrane, and their rates of turnover were significantly impacted by S availability; the turnover of SLT1 and SLT2 was proteasome dependent, while that of SULTR2 was proteasome independent. Finally, mutants identified for each of the S-deprivation-responsive transporters were used to establish their critical role in the transport of SO₄²⁻ into S-deprived cells.