Amino acids play essential roles in cell biology as regulators of metabolic pathways in addition to their role in protein and nucleic acid synthesis. Arginine in particular is a major signalling molecule inside the cell, being a precursor for both l-ornithine and nitric oxide (NO) synthesis and a key regulator of the mTORC1 pathway. In mammals, cellular arginine availability is determined by members of the solute carrier (SLC) 7 and 36 families of cationic amino acid transporters.
Several inherited disorders are linked to mutations in members of the SLC7 family, including cystinuria and lysinuric protein intolerance (LPI), both of which lead to abnormal amino acid transport, growth defects and kidney disease. Of interest to our research on drug transport, several prescription drugs, including L-DOPA and gabapentin are also recognised by members of the SLC7 family, which are expressed in the blood brain barrier and influence the transport and retention of drug molecules into the central nervous system. The ability of SLC7 transporters to mediate drug transport in the human body is a major focus of our interest in these systems.
Although in mammals CATs operate as exchangers or facilitators, in plants they are pH dependent, raising further interesting questions concerning the mechanism of coupled transport in these systems.
The SLC7 family forms part of the much larger APC superfamily of secondary active transporters, and are distantly related to the SLC36 family of proton coupled amino acid transporters or PATs. Recently, several members of the SLC36 family have been linked to regulation of the mTORC kinase and organ development in the fruit fly, suggesting these proteins may function as nutrient receptors or ‘transceptors’ in mammalian cells.
In our latest work, published in Nature Communications, we uncovered the mechanism for proton coupled amino acid transport in a prokaryotic homologue of the mammalian CATs, GkApcT (circled red above).
The crystal structure enabled the identification of a single side chain responsible for l-arginine recognition and also revealed a potential mechanism for proton coupling. The structure and associated biochemical assay data suggest that proton coupled transport is mediated through a conserved aspartate on TM6B (D237), which regulates amino acid release into the interior of the cell following de-protonation.
We are currently pursuing several lines of investigation into mammalian amino acid transport and SLC7 function in cellular physiology.