From proton microdomains to protein migration: hypothesis for ΔpH-controlled PSII relocation.

Thylakoid membranes are highly organized yet dynamic systems in which Photosystem II (PSII) complexes undergo continual assembly, repair and lateral redistribution. Here a hypothesis is proposed that local lumenal pH, generated by the trans-thylakoidal proton gradient and potentially exhibiting transient spatial heterogeneity, may contribute to the regulation of protein movement and PSII maturation within the thylakoid membrane. Five mechanistically distinct but potentially coupled routes by which ΔpH could influence protein dynamics are outlined: (i) pH-dependent modulation of lumenal protein-protein affinities; (ii) proton-sensitivity of lumenal proteases (e.g., CtpA) that gate processing and release of PSII subunits; (iii) proton-induced changes in membrane packing and stacking that alter diffusion pathways; (iv) formation of local proton microdomains that create chemically distinct niches; and (v) coupling of proton flux to electrostatic and lipid compositional shifts that bias protein localization. Together, these considerations suggest that ΔpH may act as a co-regulatory parameter integrating biochemical and biophysical cues in thylakoid organization and PSII homeostasis.