Background: LAT1 (SLC7A5) is the transport competent unit of the heterodimer formed with the glycoprotein CD98 (SLC3A2). It catalyzes antiport of His and some neutral amino acids such as Ile, Leu, Val, Cys, Met, Gln and Phe thus being involved in amino acid metabolism. Interestingly, LAT1 is over-expressed in many human cancers that are characterized by increased demand of amino acids. Therefore LAT1 was recently acknowledged as a novel target for cancer therapy. However, knowledge on molecular mechanism of LAT1 transport is still scarce. Methods: Combined approaches of bioinformatics, site-directed mutagenesis, chemical modification, and transport assay in proteoliposomes, have been adopted to unravel dark sides of human LAT1 structure/function relationships. Results: It has been demonstrated that residues F252, 5342, 035 are crucial for substrate recognition and C407 plays a minor role. 035 and C407 cannot be targeted by SH reagents. The transporter has a preferential dimeric structure and catalyzes an antiport reaction which follows a simultaneous random mechanism. Conclusions: Critical residues of the substrate binding site of LAT1 have been probed. This site is not freely accessible by molecules other than substrate. Similarly to LeuT, K+ has some regulatory properties on LAT1. General significance: The collected data represent a solid basis for deciphering molecular mechanism underlying LAT1 function. (C) 2017 Elsevier B.V. All rights reserved.