Figure 4.

Amino Acid Transport and Sensing

(A) Amino acid transporters were originally classified by their ability to translocate specific groups of amino acids across the lipid bilayer (left). However, some can activate amino-acid-dependent signaling, either in the presence or absence of transport. These so-called “transceptors” may be the precursors to modern-day receptors (right). Black arrows represent amino acid transport and green arrows signal transmission.

(B) Amino acid (AA) transport has been studied in vitro using Xenopus oocytes, facilitated by their large size (∼1,000 μm diameter) and little background transport activity; in reconstituted proteoliposomes (up to 500 μm diameter); or in human cells (∼20 μm diameter), which may contain multiple endogenous transporters. Note that in proteoliposomes, external medium may be topologically equivalent to cytosolic side of lipid bilayer, unlike the other two models.