Heteromeric amino acid transporters (HATs) are the unique example, known in all kingdoms of life, of solute transporters composed of two subunits linked by a conserved disulfide bridge. In metazoans, the heavy subunit is responsible for the trafficking of the heterodimer to the plasma membrane, and the light subunit is the transporter. HATs are involved in human pathologies such as amino acidurias, tumor growth and invasion, viral infection and cocaine addiction. However structural information about interactions between the heavy and light subunits of HATs is scarce. In this work, transmission electron microscopy and single-particle analysis of purified human 4F2hc/L-type amino acid transporter 2 (LAT2) heterodimers overexpressed in the yeast Pichia pastoris, together with docking analysis and crosslinking experiments, reveal that the extracellular domain of 4F2hc interacts with LAT2, almost completely covering the extracellular face of the transporter. 4F2hc increases the stability of the light subunit LAT2 in detergent-solubilized Pichia membranes, allowing functional reconstitution of the heterodimer into proteoliposomes. Moreover, the extracellular domain of 4F2hc suffices to stabilize solubilized LAT2. The interaction of 4F2hc with LAT2 gives insights into the structural bases for light subunit recognition and the stabilizing role of the ancillary protein in HATs.

This work was supported by Spanish Ministry of Science and Innovation Grants BIO2010-22324 (to J.F.-R.) and SAF2012- 40080-C02-01, European Commission Frame Program 7 Grant 201924 (European Drug Initative on Channels and Transporters), Fundación Ramón Areces, and the Generalitat de Catalunya Grant SGR2009-1355 (to M.P.); by University of Bern, Swiss National Science Foundation Grants 31003A_125150 and 31003A_144168; the Bern University Research Foundation; the Novartis Foundation; the Marie Curie Actions International Fellowship Program; and National Center of Competence in Research TransCure (D.F.).

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