Fig. 2.

The p _f values of membrane channels and transporters. The red and pink dots indicate pores with a known single-file region. Whether the sodium-glucose cotransporter SGLT1 (blue) has a single-file region is not entirely clear. The beta-barrel α-hemolysin (cyan) accommodates multiple water molecules in its cross-section.³⁵ Moreover, red and blue dots indicate quantitative p _f measurements in terms of protein counting. hSGLT1,³⁶ KcsA, GlpF, AQP1, AQPZ,²² AQP4,³⁷ and AQP5³⁸ were counted with fluorescence correlation spectroscopy (FCS). The bacterial cation selective gramicidin channels³⁹ were measured electrically and AQP0 (ref. 40) with electron microscopy (EM). AQP8 (ref. 41) is also permeable to ammonia and urea. Vice versa, urea transporters Ut-B and UT-2 are permeable to water.⁴²