The Ectodomains of rBAT and 4F2hc Are Fake or Orphan α-Glucosidases

Abstract

It is known that 4F2hc and rBAT are the heavy subunits of the heteromeric amino acid transporters (HATs). These heavy subunits are N-glycosylated proteins, with an N-terminal domain, one transmembrane domain and a bulky extracellular domain (ectodomain) that belongs to the α-amylase family. The heavy subunits are covalently linked to a light subunit from the SLC7 family, which is responsible for the amino acid transport activity, forming a heterodimer. The functions of 4F2hc and rBAT are related mainly to the stability and trafficking of the HATs in the plasma membrane of vertebrates, where they exert the transport activity. Moreover, 4F2hc is a modulator of integrin signaling, has a role in cell fusion and it is overexpressed in some types of cancers. On the other hand, some mutations in rBAT are found to cause the malfunctioning of the b0,+ transport system, leading to cystinuria. The ectodomains of 4F2hc and rBAT share both sequence and structure homology with α-amylase family members. Very recently, cryo-EM has revealed the structure of several HATs, including the ectodomains of rBAT and 4F2hc. Here, we analyze available data on the ectodomains of rBAT and 4Fhc and their relationship with the α-amylase family. The physiological relevance of this relationship remains largely unknown.

Structure of the human heterodimeric transporter 4F2hc-LAT2 in complex with Anticalin, an alternative binding protein for applications in single-particle cryo-EM

“…Important features of 4F2hc and rBAT heavy chains are that i) they are covalently linked to corresponding ‘light chains’, i.e., LATs, via a conserved disulfide bridge, and ii) in mammalian cells they act as chaperones for correct membrane trafficking and stabilization of the HAT 2 , 4 – 6 . The extracellular domain of the heavy chain 4F2hc shares structural similarities to bacterial glucosidases but is enzymatically inactive 7 , 8 . Light chains are polytopic membrane proteins with both termini located on the cytoplasmic side, which function as the substrate transporters 2 .…”

Structural investigation of human cystine/glutamate antiporter system xc− (Sxc−) using homology modeling and molecular dynamics

“…Since the heavy chain 4F2hc plays a critical role in stabilizing the light chain in the membrane ( Fotiadis, Kanai, and Palacín 2013 ; Rosell et al, 2014 ; Fort, Nicolàs-Aragó, and Palacín 2021 ), we added the transmembrane domain of 4F2hc to our homology models of xCT to ensure that our simulation system is stable and active in a membrane environment. Since no experimental structure of xCT was available at time of carrying out our study, this was accomplished by aligning the homology models with the available cryo-EM structure of LAT1-4F2hc, which possesses a very similar light chain structure and shares the same heavy chain.…”

How many α-amylase GH families are there in the CAZy database?

“…Notably, this subfamily was originally recognised as intermediate to the so-called oligo-1,6glucosidase and neopullulanase subfamilies [50]. Moreover, the α-amylase family GH13 contains some non-enzymatic transport proteins, rBAT and 4F2hc [77][78][79][80][81], recently suggested to represent "fake" or orphan α-glucosidases [82]. These proteins, typically found in mammals (or higher Metazoans), in most cases lost either completely or in part the GH13 catalytic residues, but still exhibited an unambiguous sequence homology in CSRs outside the regions bearing the GH13 catalytic machinery [35].…”