The light subunit of system b(o,+) is fully functional in the absence of the heavy subunit.
The heteromeric amino acid transporters are composed of a type II glycoprotein and a non-glycosylated polytopic membrane protein. System b(o,+) exchanges dibasic for neutral amino acids. It is composed of rBAT and b(o,+)AT, the latter being the polytopic membrane subunit. Mutations in either of them cause malfunction of the system, leading to cystinuria. b(o,+)AT-reconstituted systems from HeLa or MDCK cells catalysed transport of arginine that was totally dependent on the presence of one of the b(o,+) substrates inside the liposomes. rBAT was essential for the cell surface expression of b(o,+)AT, but it was not required for reconstituted b(o,+)AT transport activity. No system b(o,+) transport was detected in liposomes derived from cells expressing rBAT alone. The reconstituted b(o,+)AT showed kinetic asymmetry. Expressing the cystinuria-specific mutant A354T of b(o,+)AT in HeLa cells together with rBAT resulted in defective arginine uptake in whole cells, which was paralleled by the reconstituted b(o,+)AT activity. Thus, subunit b(o,+)AT by itself is sufficient to catalyse transmembrane amino acid exchange. The polytopic subunits may also be the catalytic part in other heteromeric transporters.
Reig
Núria
N
Departament de Bioquimica i Biologia Molecular, Facultat de Biologia, Universitat de Barcelona, Avda Diagonal 645, Barcelona E-08028, Spain.
Chillarón
Josep
J
Bartoccioni
Paola
P
Fernández
Esperanza
E
Bendahan
Annie
A
Zorzano
Antonio
A
Kanner
Baruch
B
Palacín
Manuel
M
Bertran
Joan
J
D026905:Amino Acid Transport Systems / Q000235:genetics / Q000502:physiology*; D026923:Amino Acid Transport Systems, Basic; D000818:Animals; D001120:Arginine / Q000378:metabolism; D001692:Biological Transport / Q000502:physiology; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D003553:Cystine / Q000378:metabolism; D003555:Cystinuria / Q000235:genetics; D004285:Dogs; D006367:HeLa Cells; D006801:Humans; D007668:Kidney / Q000166:cytology; D007930:Leucine / Q000378:metabolism; D008081:Liposomes / Q000737:chemistry / Q000378:metabolism; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism*; D021122:Protein Subunits
D026905:Amino Acid Transport Systems; D026923:Amino Acid Transport Systems, Basic; D002352:Carrier Proteins; D008081:Liposomes; D008562:Membrane Glycoproteins; D021122:Protein Subunits; C499489:SLC7A9 protein, human; D003553:Cystine; D001120:Arginine; D007930:Leucine