The E. coli water channel aquaporin Z (AqpZ) is widely used in biomimetic membranes for water purification. However, the rationale for its lipid-dependent water permeability and its relevance for biomimetic membranes remain unclear. Cardiolipin (CDL), a known AqpZ-binding lipid, increases permeability by more than twofold in a cooperative manner. To investigate possible CDL-induced structural changes in AqpZ, we determined high resolution (<2 Å)AqpZ structures using single-particle cryo-electron microscopy in nanodiscs of controllable lipid composition. Unexpectedly, these structures were indistinguishable in synthetic dioleoyl-glycero-3-phosphocholine (DOPC) and in E. coli lipids containing up to 15% CDL, despite clear functional differences. We hypothesized that this structural invariance likely is due to the isolation of individual AqpZ tetramers within nanodiscs, whereas water permeability may be due to lipid-dependent formation of hypothetical multi-tetramer ensembles (MTEs) in membranes. Indeed, we found that crosslinking and FRAP experiments were consistent with reduced AqpZ tetramer clustering and increased mobility in membranes containing added CDL or in native E. coli lipids. Further, extensive ordered arrays were observed by cryo-electron microscopy in untreated DOPC AqpZ proteoliposomes, whereas CDL supplementation disrupted these arrays. These results suggest that CDL promotes tetramer dispersion from MTEs that correlates with enhanced permeability. Controlling MTE formation may provide a strategy to optimize aquaporin-based biomimetic membranes. • Aquaporin Z from E. coli (ApqZ) was obtained in lipid nanodiscs at high resolution. • Structure of the AqpZ tetramer is independent from lipid composition. • Multi-tetramer AqpZ ensembles exist in lipid membranes. • Cardiolipin activates AqpZ and correlates with dispersion of these ensembles. • Water permeability in biomimetic membranes may be increased by this mechanism.
Surya et al. (Sun,) studied this question.