SYSTEM Y(- THE BROAD SCOPE AND CATION MODULATED AMINO-ACID TRANSPORTER()L )

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 translo cate cationic and neutral amino acids, and (b) its specificity varies depending on the ionic composition of the medium. In Na+ medium, the h alf-saturation constant far L-lysine influx was 9.5 +/- 0.67 mu M and the inhibition constant (K-i) for L-leucine was 10.7 +/- 0.72 mu M. L- Leucine is the neutral amino acid that binds more powerfully, whereas smaller analogues, such as L-alanine and L-serine interact less strong ly (the corresponding inhibition constants were K-1,K-Ala, 0.62 +/- 0. 11 mM; K-i,K-Ser, 0.49 +/- 0.08 mM). In the presence of K+, the carrie r functions as a cationic amino acid specific carrier, but Li+ is able to substitute for Na+ facilitating neutral amino acid binding. The ef fect of the inorganic cations is restricted to the recognition of neut ral amino acids; translocation occurs at similar rates in the presence of Na+, K+ and Li+. The only structural feature that appears to impai r translocation is bulkiness and substrates with half-saturation const ants differing by more than 100-fold translocate at the same rate. Thi s suggests that translocation is largely independent of the forces of interaction between the substrate and the carrier site. System y(+)L a ctivity 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 do main and it remains to be clarified whether it is part of the transpor ter or an activator protein.