Aquaporins Are Proteins That Form Water Channels in Certain Membranes

In certain cells (eg, red cells, cells of the collecting duc-tules of the kidney), the movement of water by simple

als indicate the four subunits of each channel and the Arabic numerals the a-helical transmembrane domains of each subunit. The actual pores through which the ions pass are not shown but are formed by apposition of the various subunits. The specific areas of the subunits involved in the opening and closing of the channels are also not indicated. (After WK Catterall. Modified and reproduced from Hall ZW: An

Introduction to Molecular Neurobiology. Sinauer, 1992.)

als indicate the four subunits of each channel and the Arabic numerals the a-helical transmembrane domains of each subunit. The actual pores through which the ions pass are not shown but are formed by apposition of the various subunits. The specific areas of the subunits involved in the opening and closing of the channels are also not indicated. (After WK Catterall. Modified and reproduced from Hall ZW: An

Introduction to Molecular Neurobiology. Sinauer, 1992.)

Table 41-4. Some properties of ion channels.

• They are composed of transmembrane protein subunits.

• Most are highly selective for one ion; a few are nonselective.

• They allow impermeable ions to cross membranes at rates approaching diffusion limits.

• They can permit ion fluxes of 106-107/s.

• Their activities are regulated.

• The two main types are voltage-gated and ligand-gated.

• They are usually highly conserved across species.

• Most cells have a variety of Na+, K+, Ca2+, and CI- channels.

• Mutations in genes encoding them can cause specific diseases.1

• Their activities are affected by certain drugs.

1Some diseases caused by mutations of ion channels are briefly discussed in Chapter 49.

TOI wm

I 1 Lipid j bilayer

Uniport

Symport

Antiport

Cotransport

Figure 41-10. Schematic representation of types of transport systems. Transporters can be classified with regard to the direction of movement and whether one or more unique molecules are moved. (Redrawn and reproduced, with permission, from Alberts B et al: Molecular Biology of the Cell. Garland, 1983.)

diffusion is augmented by movement through water channels. These channels are composed of tetrameric transmembrane proteins named aquaporins. At least five distinct aquaporins (AP-1 to AP-5) have been identified. Mutations in the gene encoding AP-2 have been shown to be the cause of one type of nephrogenic diabetes insipidus.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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