Antiporter controls intracellular pH
As noted in a 2002 article, "Proton fluxes at the plasma membrane are regulated by several families of ion exchangers, including the Na+/H+ exchangers(NHEs) and HCO3 transporters, such as the Na+/HCO-3- cotransporters (NBCs), Cl−/HCO−3 exchangers (AEs), and the Na+ driven Cl−/HCO−3- exchanger (NDAE). 3
NHEs: Antiporters most important to pH
NHEs are the most widely discussed antiporters in relation to pH. There are six members of the NHE family and they are evolutionarily conserved. NHE1 is responsible for "pH and cell-volume homeostasis." It is made up of approximately 820 amino acids and is similar in structure to other ion exchangers. The most highly conserved regions of this protein are TM 6 and 7.
While NHEs play a role in shaping intracellular pH, they themselves are regulated by pH: "NHE1 is highly sensitive to changes in intracellular H+ (H+i ), such that reduced pHi allosterically activates the protein (46). Hence, the kinetics of NHE activity in response to H+ is more complex than that observed for extracellular substrates. NHE1, NHE2, and NHE3 are extremely sensitive to low pHi. At physiological pH, NHE1 and NHE2 are essentially inactive, but they are rapidly activated upon reduction in pH (38, 46), whereas NHE3 is active at neutral pHi because it has a higher affinity for H+," 3
NHE1 is regulated by a number of other intracellular complexes. It has been noted that its C-terminus is especially important in determining its sensitivity to changes in pH.
Below is a graphic of the C-terminus of NHE1 and the predicted binding sites:
Other ion exchangers noted to have a key role in pH are AE1 and NBC1.
The NHE family of genes is referred to as NHX in plants. This family includes NHX1, NHX2, etc.
Actual pH of blueberries
pH of berry:
The pH of blueberry leaves has actually been found to be substantially lower than that of other fruit, such as strawberry and raspberry. 4 Additionally, younger blueberry plants have leaves with a lower pH than older plants. This is the reverse of what has been found in other cultivated plants.
Question this raises: What role do the low-pH leaves play in maintaining the high pH of the berry?
How should we use this information in our studies?
In studying the blueberry genome, we should look for these key pH-regulating proteins and note any differences.
- Are the same regions highly conserved? - Are key binding regions different in blueberry? - Is the C-terminus of NHE1 in blueberries any different, such that it reacts differently to changes in pH?
Possible wet lab activity: if the berry and the leaves have such different pH's, what if we tested for the amount of pH-regulation-related proteins in the berry vs. the leave, to figure out which proteins might be most responsible for the pH of the berry?