Abstract
Aquaporins are integral membrane proteins that facilitate the diffusion of water and other small, uncharged solutes across the cellular membrane and are widely distributed in organisms from humans to bacteria. However, the characteristics of prokaryotic aquaporins remain largely unknown. We investigated the distribution and sequence characterization of aquaporins in prokaryotic organisms and summarized the transport characteristics, physiological functions, and regulatory mechanisms of prokaryotic aquaporins. Aquaporin homologues were identified in 3315 prokaryotic genomes retrieved from the Kyoto Encyclopedia of Genes and Genomes (KEGG) database, but the protein clustering pattern is not completely congruent with the phylogeny of the species that carry them. Moreover, prokaryotic aquaporins display diversified aromatic/arginine constriction region (ar/R) amino acid compositions, implying multiple functions. The typical water and glycerol transport characterization, physiological functions, and regulations have been extensively studied in Escherichia coli AqpZ and GlpF. A Streptococcus aquaporin has recently been verified to facilitate the efflux of endogenous H2O2, which not only contributes to detoxification but also to species competitiveness, improving our understanding of prokaryotic aquaporins. Furthermore, recent studies revealed novel regulatory mechanisms of prokaryotic aquaporins at post-translational level. Thus, we propose that intensive investigation on prokaryotic aquaporins would extend the functional categories and working mechanisms of these ubiquitous, intrinsic membrane proteins.
Highlights
Aquaporins are integral membrane proteins that form tetramers and facilitate the diffusion of water and some small, uncharged solutes across cellular membranes [1,2]
Aquaporins belong to the major intrinsic protein (MIP) family that is comprised of more than 1700 integral membrane proteins [6,7]
Based on the primary sequences, they are classified into the water-selective aquaporins (AQPs), glycerol-transporting aquaglyceroporins (GLPs) and unorthodox aquaporins; the third subfamily is only present in animals with unverified substrate permeability [3,8]
Summary
Aquaporins are integral membrane proteins that form tetramers and facilitate the diffusion of water and some small, uncharged solutes across cellular membranes [1,2]. A number of other uncharged solutes or gases are reported to cross the cellular membrane via aquaporin channels, which include urea, ammonia, hydrogen peroxide (H2 O2 ), carbon dioxide (CO2 ), metalloids, nitric oxide (NO), malate-aluminum, and even ions [20,21,22,23,24,25,26] Because these small molecules are involved in a variety of metabolic processes or function as signal molecules [13,27], aquaporins have been verified to play important roles in organisms in stress response, growth, development, and disease processes, including tumorigenesis and metabolic disorders [28,29,30,31]. By using genome sequence information and homology analysis, we investigated the distribution and sequence characteristics of aquaporin homologues in prokaryotic species, and summarized the findings of recent investigations on prokaryotic aquaporins in relation to substrate transport, physiological functions, regulatory mechanisms, and the factors contributing to the stability of aquaporin tetramers
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