Plasma membrane H+-ATPases
The H+-ATPase located in the plasma membrane is a crucial primary active transporter in plant and fungal cells. Its primary function is the outward active transport of protons (H+) across the plasma membrane, which establishes essential electrochemical gradients that power the cell.
Functional Significance
The active pumping of H+ out of the cytosol generates two major gradients that are critical for cellular function:
pH Gradient: The outside of the cell becomes more acidic (lower pH) relative to the cytosol (cytosolic pH is typically 7.0 to 7.5).
Electrical Potential (Membrane Potential): Because positive charges (H+) are pumped out, the inside of the cell becomes more negative relative to the outside, generating a negative membrane potential.
These two components together form the Proton Motive Force (PMF), which is then harnessed by secondary active transport proteins (cotransporters) to drive the uphill transport of many other substances, including ions and uncharged solutes (like sugars).
Roles of Plasma Membrane H+-ATPase Activity:
Nutrient Movement: High expression in cells involved in nutrient uptake (e.g., root endodermis) and movement toward developing seeds.
Cytosolic pH Regulation: Maintaining the correct internal pH.
Cell Turgor Control: This turgor pressure drives processes like cell growth, stomatal opening (in guard cells, where specific isoforms are expressed), and organ movement (leaf and flower movement).
Molecular Characteristics (P-type ATPases)
This pump is responsible for actively transporting H+ out of the cytosol across the plasma membrane.
Classification: Belongs to the P-type ATPase family.
Catalytic Mechanism: Characterized by forming a phosphorylated aspartic acid intermediate during the ATP hydrolysis cycle.
Structure (Based on Yeast Model) and regulation:
The protein spans the membrane multiple times, possessing about ten membrane-spanning domains.
These domains contribute to forming the pathway through which protons (H+) are translocated across the membrane.
The catalytic domain (where ATP is hydrolyzed) is located on the cytosolic face of the membrane. This domain contains the critical aspartic acid residue that becomes phosphorylated.
A specialized autoinhibitory domain is located at the C-terminal end of the polypeptide chain.
This regulatory domain normally keeps the enzyme inactive.
Its regulatory action can be overridden by phosphorylation, which recruits 14-3-3 proteins, leading to the displacement of the autoinhibitory domain and pump activation.
Fusicoccin: This fungal toxin acts as a powerful activator by increasing the affinity of the 14-3-3 proteins for the phosphorylated region, even without phosphorylation occurring. This activation can be so strong that it causes irreversible stomatal opening, wilting, and plant death.
Vacuolar H+-ATPase (V-ATPase)
This pump is located on the tonoplast (the membrane surrounding the central vacuole) and pumps H+ into the vacuole.
Classification: It is more closely related to the F-ATPases found in mitochondria and chloroplasts.
Catalytic Mechanism: Does not form a phosphorylated intermediate during ATP hydrolysis.
Structure: It is a very large enzyme complex (approximately 750 kDa) composed of multiple subunits organized into two main complexes:
V1 Complex (Peripheral): Located outside the vacuole (on the cytosolic side), this complex is responsible for ATP hydrolysis.
V0 Complex (Integral Membrane): Embedded within the tonoplast, this complex is responsible for H+ translocation across the membrane.
Proposed Operation: Due to similarities with F-ATPases, the V-ATPase is assumed to operate like a tiny rotary motor.
Electrogenicity: Like the plasma membrane pump, it is electrogenic, resulting in the vacuole being typically 20 to 30 mV more positive than the cytosol. To maintain electrical neutrality while pumping H+, anions (Cl- or malate2-) are simultaneously transported into the vacuole via anion channels.
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