Post-translational modification acts as a digital like switch influencing AtPIP2;1 water and cation permeability

Area of Science:

• Plant molecular biology
• Membrane transport proteins
• Biophysics

Background:

• Plant aquaporins (AQPs) are membrane proteins primarily known for water transport.
• Some AQPs, like Arabidopsis thaliana PIP2;1 (AtPIP2;1), exhibit broader transport functions, including cations.
• Mechanisms regulating AtPIP2;1 selectivity for water versus ions are not fully understood.

Purpose of the Study:

• To investigate the role of phosphorylation sites in regulating AtPIP2;1 selectivity.
• To determine how mutations mimicking phosphorylation/dephosphorylation affect water and cation transport.
• To elucidate the gating mechanism controlling AtPIP2;1 permeability.

Main Methods:

• Site-directed mutagenesis of four serine phosphorylation sites in AtPIP2;1.
• Expression of wild-type and mutant AtPIP2;1 in Xenopus laevis oocytes.
• Electrophysiological analysis of water and ion (cation) conductance.

Main Results:

• Mutations at phosphorylation sites act as a 'selectivity switch,' modulating AtPIP2;1 permeability between cations and water.
• A reciprocal relationship exists between cation and water transport, suggesting a gated pore mechanism.
• Specific mutants (e.g., S194A/S280D/S283D) showed high cation conductance with distinct selectivity, while others maximized water transport.

Conclusions:

• Post-translational regulation via phosphorylation provides AtPIP2;1 flexibility to switch transport roles.
• This dynamic switching is vital for maintaining water and ion homeostasis in plants.
• The findings reveal a novel regulatory mechanism for aquaporin function.