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Westlake University Published Research Results on Human Heteromeric Amino Acid Transporter

Shan XU
18, 2020

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 Westlake University published a discovery on human heteromeric amino acid transporter b0,+AT-rBAT on Science Advances, revealing how cystinuria is caused on the molecule level on April 16th.

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Amino acids are pieces that form the proteins which make up most of our body parts and functions, whereas amino acid transporters are carrying these important molecules through the membrane of our cells to the needed areas. But because of some gene defects, those transporters may be wrongly programmed and cause unwanted accumulations of amino acids like stones formed with the amino acid cystine in the kidneys, bladder, and ureters, an inherited disease known as cystinuria. Research on the molecular structure of the transporter b0,+AT-rBAT allows us to better understand cystinuria at the molecular level.

Dr.Renhong Yan, PhD candidate Yaning Li, Dr.Yi Shi, listed as first authors of the paper, and the principle investigators Qiang Zhou and Jing Huang, listed as the correspondent authors, have studied the structure of these important transporters through cryogenic electron microscopy (cryo-EM), an advanced tool that allows us to view and study the structure of various biological molecules.

The human amino acid transporter b0,+AT-rBAT is one of the heteromeric amino acid transporters (HAT).“Heteromeric” just means that the transporter consists of more than one kind of structural subunit. The b0,+AT-rBAT transporter comprises two different subunits, a heavy chain and a light chain. The b0,+AT (SLC7A9) is a representative light chain of HATs, forming heterodimer with rBAT, a heavy chain which mediates the membrane trafficking of b0,+AT and maintains its stability.

The b0,+AT part is usually found in kidneys and small intestines. Mutations in b0,+AT or rBAT cause cystinuria. The overall prevalence of cystinuria is approximately 1 in 7,000 neonates. Thus,the disease counts as a rare disease. But the few unlucky patients, who suffer from cystinuria, must often endure pain caused by urinary stones blocking the urinary tract, urinary tract infections (UTI), or kidney failures caused by cystine stones. The research on b0,+AT-rBAT uncovered the molecule mechanism of cystinuria, the structure and function of this complex, which helps us understand the disease better and treat it more effectively.


[Structural determination of the transporter b0,+AT-rBAT]

This research revealed the cryo-EM structure of b0,+AT-rBAT for the first time ever. The structure shows that the b0,+AT forms the heterodimer with rBAT first. Then, two heterodimers form another dimer through the interaction of rBat.


The in vitro transport experiment shows that rBAT is obligatory to the transportation of b0,+AT. This is consistent to the LAT1-4F2hc researched by Qiang Zhou’s lab in 2019. LAT1-4F2hc is also a kind of HAT, in which 4F2hc is obligatory to the transportation of LAT1. 


Here, we report the cryo-EM structure of the human b0,+AT-rBAT complex alone and in complex with arginine substrate at resolution of 2.7 and 2.3 Å, respectively.


If b0,+AT-rBAT was a ship on the membrane, then the arginine substrate would be the cargo. Through understanding b0,+AT-rBAT and the structure of its substrate, the team found out how the “cargo” was loaded onto the “ship”.


The overall structure of b0,+AT-rBAT exists as a dimer of heterodimer consistent with the previous study. A ligand molecule is bound to the substrate binding pocket, near which an occluded pocket is identified, to which we found is important for the substrate transport.


The paper also reported the cryo-EM structure of the human b0,+AT-rBAT complex alone and in complex with arginine substrate. If we are to see b0,+AT-rBAT as a ship on the membrane, then the arginine would be the cargo. Through learning b0,+AT-rBAT and the structure of its substrate, the team found out how the “cargo” was loaded on the “ship”.

[A proposed model for substrate transport mediated by the transporter b0,+AT-rBAT complex.]


The team also identified a transport mediating area near the binding pocket. Through running experiments on point mutation and isotope, they proved that this mediating area was crucial for the substrate transport. Jing Huang’s lab at Westlake University verified the importance using molecular simulation. 


Thanks to the high resolution structure of b0,+AT-rBAT, the researchers can accurately locate the mutated points and verify them in experiments outside the living organism. The results show that the mutation in the key points on b0,+AT-rBAT affects the activity of amino acid transportation and causes cystinuria. 


This is yet another achievement of Qiang Zhou’s lab at Westlake University on human heteromeric amino acid transporters after the 2019 publication on the cryo-EM structure of LAT1-4F2hc and the 2020 publication on the cryo-EM structure of the complex between the neutral amino acid transporter B0AT1 (SLC6A19) and ACE2, an enzyme that the coronavirus uses to enter the human cell.