1. Introduction 362

1.1 The elusive water pores 362

1.2 CHIP28 362

2. Studies on AQP-1 363

2.1 Expression of AQP1 cDNA in Xenopus oocytes 363

2.2 Reconstitution of purified AQP1 into artificial lipid bilayers 364

2.3 Structural information deduced from the primary sequence 365

2.4 Evolution and mammalian AQPs 365

3. Chronological overview over AQP structures 368

3.1 AQP1 – the red blood cell water pore 368

3.2 GlpF – the E. coli glycerol facilitator 371

3.3 AQPZ – the E. coli water pore 372

3.4 AQP0 – the lens-specific aquaporin 373

3.5 AQP4 – the main aquaporin in brain 377

3.6 SoPiP2;1 – a plant aquaporin 379

3.7 AQPM – an archaeabacterial aquaporin 379

4. Proton exclusion 380

5. Substrate selectivity 382

6. Pore regulation 385

6.1 Hormonal regulation of AQP trafficking 385

6.2 Influence of pH on AQP water conduction 386

6.3 Regulation of AQP pore conductance by protein binding 387

6.4 Pore closure by conformational changes in the AQP0 pore 388

7. Unresolved questions 390

8. Acknowledgments 390

9. References 391

The ubiquitous members of the aquaporin (AQP) family form transmembrane pores that are either exclusive for water (aquaporins) or are also permeable for other small neutral solutes such as glycerol (aquaglyceroporins). The purpose of this review is to provide an overview of our current knowledge of AQP structures and to describe the structural features that define the function of these membrane pores. The review will discuss the mechanisms governing water conduction, proton exclusion and substrate specificity, and how the pore permeability is regulated in different members of the AQP family.