Figure 11–10 Transporters are built from inverted repeats. (A) LeuT, a bacterial leucine/Na+ symporter related to human neurotransmitter transporters, such as the serotonin transporter, is shown. The core of the transporter is built from two bundles, each composed of five α helices (blue and yellow). The helices shown in gray differ among members of this transporter family and are thought to play regulatory roles, which are specific to a particular transporter. (B) Both core helix bundles are packed in a similar arrangement (shown as a hand, with the broken helix as the thumb), but the second bundle is inverted with respect to the first. The transporter’s structural pseudosymmetry reflects its functional symmetry: the transporter can work in either direction, depending on the direction of the ion gradient. (Adapted from
K.R.
Vinothkumar and R. Henderson,
Q.
Rev. Biophys. 43:65–158, 2010. With permission from Cambridge University Press. PDB code: 3F3E.) cytosol. A Na+-independent Cl––HCO3 exchanger in the membrane of red blood cells (called band 3 protein—see Figure 10–38) facilitates the quick discharge of CO2 (as HCO3–) as the cells pass through capillaries in the lung.
The intracellular pH is not entirely regulated by transporters in the plasma membrane: ATP-driven H+ pumps are used to control the pH of many intracellular compartments. As discussed in Chapter 13, H+ pumps maintain the low pH in lysosomes, as well as in endosomes and secretory vesicles. These H+ pumps use the energy of ATP hydrolysis to pump H+ into these organelles from the cytosol.
An Asymmetric Distribution of Transporters in Epithelial Cells Underlies the Transcellular Transport of Solutes
In epithelial cells, such as those that absorb nutrients from the gut, transporters are distributed nonuniformly in the plasma membrane and thereby contribute to the transcellular transport of absorbed solutes. By the actions of the transporters in these cells, solutes are moved across the epithelial cell layer into the extracellular fluid from where they pass into the blood. As shown in Figure 11–11, Na+-linked symporters located in the apical (absorptive) domain of the plasma membrane actively transport nutrients into the cell, building up substantial concentration gradients for these solutes across the plasma membrane. Uniporters in the basal and lateral (basolateral) domains allow the nutrients to leave the cell passively down these concentration gradients.
In many of these epithelial cells, the plasma membrane area is greatly increased by the formation of thousands of microvilli, which extend as thin, fingerlike projections from the apical surface of each cell. Such microvilli can increase the total absorptive area of a cell as much as 25-fold, thereby enhancing its transport capabilities.
As we have seen, ion gradients have a crucial role in driving many essential transport processes in cells. Ion pumps that use the energy of ATP hydrolysis establish and maintain these gradients, as we discuss next.
transporter mediating passive transport of glucose