The structural and functional units of heteromeric amino acid transporters -: The heavy subunit rBAT dictates oligomerization of the heteromeric amino acid transporters

Heteromeric amino acid transporters are composed of a catalytic light subunit and a heavy subunit linked by a disulfide bridge. We analyzed the structural and functional units of systems b(0,+) and x(C)(-), formed by the heterodimers b(0,+) AT-rBAT and xCT-4F2hc, respectively. Blue Native gel electrophoresis, cross-linking, and fluorescence resonance energy transfer in vivo indicate that system b(0,+) is a heterotetramer [b(0,+) AT-rBAT](2), whereas xCT-4F2hc seems not to stably or efficiently oligomerize. However, substitution of the heavy subunit 4F2hc for rBAT was sufficient to form a heterotetrameric [xCT-\rBAT](2) structure. The functional expression of concatamers of two light subunits ( which differ only in their sensitivity to inactivation by a sulfhydryl reagent) suggests that a single heterodimer is the functional unit of systems b(0,+) and x(C)(-).