FIGURE 6.

Model depicting potential regulatory function of SULTR1;2-OASTL interaction. A and B, high SO₄²⁻ condition. A decline in cellular energy status compromises the ability of cells to reduce SO₄²⁻ in plastids and/or mitochondria, which favors low S²⁻ and high OAS in the cytosol (in a high SO₄²⁻ environment). Elevated OAS leads to STAS-OASTL interaction. The interaction may negatively affect SULTR1;2 activity and positively affect OASTL activity; OASTL remains active when free in the cytoplasm, has increased activity when bound to SULTR1;2, and becomes inactive when complexed with SAT. B, if S²⁻ synthesis keeps pace with OAS production, the OAS is consumed in the formation of cysteine, OASTL is released from the STAS domain, and SULTR1;2 is free to transport SO₄²⁻. Upon re-binding of OAS to OASTL, the situation depicted in A is re-established. Repeated cycling between (A and B) coordinates SULTR1;2 activity with the ability of the cell to reduce SO₄²⁻. C, low SO₄²⁻ condition. When SO₄²⁻ is low, a conformational change in SULTR1;2 may lessen the STAS-OASTL interaction, allowing higher rates of SO₄²⁻ transport activity despite potentially high OAS levels. Furthermore, low SO₄²⁻ conditions favor the synthesis of SULTR1;1, a high affinity transporter that is insensitive to OASTL regulation. Fdred, reduced ferredoxin; GSH, glutathione.