System y+L: the broad scope and cation modulated amino acid transporter.
The properties are discussed of system y+L, a broad scope amino acid transporter which was first identified in human erythrocytes. System y+L exhibits two distinctive properties: (a) it can bind and translocate cationic and neutral amino acids, and (b) its specificity varies depending on the ionic composition of the medium. In Na+ medium, the half-saturation constant for L-lysine influx was 9.5 +/- 0.67 microM and the inhibition constant (Ki) for L-leucine was 10.7 +/- 0.72 microM. L-Leucine is the neutral amino acid that binds more powerfully, whereas smaller analogues, such as L-alanine and L-serine interact less strongly (the corresponding inhibition constants were Ki,Ala, 0.62 +/- 0.11 mM; Ki,Ser, 0.49 +/- 0.08 mM). In the presence of K+, the carrier functions as a cationic amino acid specific carrier, but Li+ is able to substitute for Na+ facilitating neutral amino acid binding. The effect of the inorganic cations is restricted to the recognition of neutral amino acids; translocation occurs at similar rates in the presence of Na+, K+ and Li+. The only structural feature that appears to impair translocation is bulkiness and substrates with half-saturation constants differing by more than 100-fold translocate at the same rate. This suggests that translocation is largely independent of the forces of interaction between the substrate and the carrier site. System y+L activity has been observed in Xenopus laevis oocytes injected with the cRNA for the heavy chain of the 4F2 human surface antigen. 4F2hc is an integral membrane protein with a single putative membrane-spanning domain and it remains to be clarified whether it is part of the transporter or an activator protein.
Devés
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Programa de Fisiología y Biofísica, Facultad de Medicina, Universidad de Chile, Santiagó, Chile. rdeves@bitmed.med.uchile.cl
Angelo
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Rojas
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D026905:Amino Acid Transport Systems; D000596:Amino Acids / Q000097:blood*; D000818:Animals; D001692:Biological Transport / Q000502:physiology; D002352:Carrier Proteins / Q000096:biosynthesis / Q000097:blood* / Q000235:genetics / Q000502:physiology; D002414:Cations, Monovalent / Q000097:blood* / Q000494:pharmacology; D004912:Erythrocytes / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D008564:Membrane Potentials; D011485:Protein Binding; D014982:Xenopus laevis
D026905:Amino Acid Transport Systems; D000596:Amino Acids; D002352:Carrier Proteins; D002414:Cations, Monovalent