Integration of computational modeling with membrane transport studies reveals new insights into amino acid exchange transport mechanisms.
Uptake of system L amino acid substrates into isolated placental plasma membrane vesicles in the absence of opposing side amino acid (zero-trans uptake) is incompatible with the concept of obligatory exchange, where influx of amino acid is coupled to efflux. We therefore hypothesized that system L amino acid exchange transporters are not fully obligatory and/or that amino acids are initially present inside the vesicles. To address this, we combined computational modeling with vesicle transport assays and transporter localization studies to investigate the mechanisms mediating [(14)C]L-serine (a system L substrate) transport into human placental microvillous plasma membrane (MVM) vesicles. The carrier model provided a quantitative framework to test the 2 hypotheses that l-serine transport occurs by either obligate exchange or nonobligate exchange coupled with facilitated transport (mixed transport model). The computational model could only account for experimental [(14)C]L-serine uptake data when the transporter was not exclusively in exchange mode, best described by the mixed transport model. MVM vesicle isolates contained endogenous amino acids allowing for potential contribution to zero-trans uptake. Both L-type amino acid transporter (LAT)1 and LAT2 subtypes of system L were distributed to MVM, with L-serine transport attributed to LAT2. These findings suggest that exchange transporters do not function exclusively as obligate exchangers.
['Widdows|Kate L|KL|', 'Panitchob|Nuttanont|N|', 'Crocker|Ian P|IP|', 'Please|Colin P|CP|', 'Hanson|Mark A|MA|', 'Sibley|Colin P|CP|', 'Johnstone|Edward D|ED|', 'Sengers|Bram G|BG|', 'Lewis|Rohan M|RM|', 'Glazier|Jocelyn D|JD|']
D027182:Amino Acid Transport System y+ D000596:Amino Acids D001692:Biological Transport D015153:Blotting, Western D002250:Carbon Radioisotopes D002462:Cell Membrane D003198:Computer Simulation D005260:Female D005455:Fluorescent Antibody Technique D027301:Fusion Regulatory Protein 1, Light Chains D006801:Humans D027283:Large Neutral Amino Acid-Transporter 1 D008871:Microvilli D008954:Models, Biological D010920:Placenta D011247:Pregnancy D012694:Serine D022161:Transport Vesicles
facilitated transport LAT2 (SLC7A8) antiporters overshoot phenomena