Abstract

Culex quinquefasciatus Cqm1 protein acts as the receptor for Lysinibacillus sphaericus mosquito-larvicidal binary (BinAB) toxin that is used worldwide for mosquito control. We found amino acid transporter protein, rBAT, as phylogenetically closest Cqm1 homolog in humans. The present study reveals large evolutionary distance between Cqm1 and rBAT, and rBAT ectodomain lacks the sequence motif which serves as binding-site for the BinAB toxin. Thus, BinAB toxin can be expected to remain safe for humans. rBAT (heavy subunit; SLC3A1) and catalytic b^0,+AT (light subunit; SLC7A9), linked by single disulfide bond, mediate renal reabsorption of cystine and dibasic amino acids in Na⁺ independent manner. Mutations in rBAT cause type I Cystinuria disease which shows global prevalence, and rBAT can be thought as an important pharmacological target. However, 3D structures of rBAT and b^0,+AT, the two components of b^0,+ heteromeric amino acid transporter systems, are not available. We constructed a reliable homology model of rBAT using Cqm1 coordinates and that of transmembrane b^0,+AT subunit using LAT1 coordinates. Mapping of pathogenic mutations onto rBAT ectodomain revealed their scattered distribution throughout the rBAT protein. Further, our computational simulations-based scoring of several known deleterious mutations of rBAT revealed that mutations those do not compromise the protein fold and stability, are localized on the same face of the molecule. These residues are expected to interact with the b^0,+AT transporter. The present study thus identifies druggable sites on rBAT that could be targeted for the treatment of type I Cystinuria.

Communicated by Ramaswamy H. Sarma

摘要

Culex quinquefasciatus Cqm1 蛋白充当球形赖氨酸杆菌杀蚊幼虫二元 (BinAB) 毒素的受体，该毒素在全球范围内用于控制蚊子。我们发现氨基酸转运蛋白 rBAT 是人类系统发育最接近的 Cqm1 同源物。本研究揭示了 Cqm1 和 rBAT 之间的大进化距离，并且 rBAT 胞外域缺乏作为 BinAB 毒素结合位点的序列基序。因此，可以预期 BinAB 毒素对人类保持安全。rBAT（重亚基；SLC3A1）和催化性 b ^0,+ AT（轻亚基；SLC7A9），通过单个二硫键连接，介导 Na ⁺中胱氨酸和二元氨基酸的肾重吸收独立的方式。rBAT 突变导致 I 型胱氨酸尿症，该病在全球范围内流行，而 rBAT 可被认为是一个重要的药理学靶点。然而，rBAT 和 b ^0,+ AT（b ^0,+异聚氨基酸转运系统的两个组成部分）的3D 结构不可用。我们使用 Cqm1 坐标和跨膜 b ^0,+坐标构建了一个可靠的 rBAT 同源模型使用 LAT1 坐标的 AT 亚基。将致病突变映射到 rBAT 胞外域揭示了它们在整个 rBAT 蛋白中的分散分布。此外，我们对几个已知的 rBAT 有害突变的基于计算模拟的评分显示，那些不会损害蛋白质折叠和稳定性的突变位于分子的同一面上。预计这些残基会与 b ^0,+ AT 转运蛋白相互作用。因此，本研究确定了 rBAT 上可靶向治疗 I 型胱氨酸尿症的药物位点。

由 Ramaswamy H. Sarma 交流