AqpZ

AqpZ, or Aquaporin Z, is a water channel protein found in bacteria, most notably Escherichia coli. It belongs to the aquaporin superfamily and functions to facilitate rapid, selective water movement across the cell membrane in response to osmotic gradients. AqpZ proteins are typically present as tetramers in the inner membrane, with each monomer forming its own water pore. Each subunit contains six transmembrane helices and two highly conserved NPA (asparagine-proline-alanine) motifs that contribute to pore formation and proton exclusion. The narrowest part of the pore is the ar/R constriction, formed by residues from the surrounding helices, which helps define water selectivity and prevents proton leakage.

Functionally, AqpZ mediates fast water transport to help regulate cell volume during hypo- and hypertonic stress. The channel is primarily water-selective and shows little to no permeability to glycerol or other solutes, distinguishing it from aquaglyceroporins. In vivo activity can be influenced by osmotic conditions and membrane composition, and structural studies have illuminated the mechanisms that enable high water flux while maintaining proton impermeability.

Structural studies, including X-ray crystallography, have revealed the tetrameric arrangement and the hourglass-shaped pore architecture of each monomer. These insights have advanced the understanding of aquaporin function, including how selectivity and gating contribute to efficient water transport and proton exclusion.

AqpZ homologues are distributed across many Gram-negative bacteria and some archaea, indicating a conserved role in bacterial osmoregulation. Ongoing research continues to explore its regulation under different environmental conditions and its potential applications in biotechnology and synthetic biology.