Hartnup disorder (OMIM 234500) is an autosomal recessive abnormality of renal and gastrointestinal neutral amino acid transport noted for its clinical variability. We localized a gene causing Hartnup disorder to chromosome 5p15.33 and cloned a new gene, SLC6A19, in this region. SLC6A19 is a sodiumdependent and chloride-independent neutral amino acid transporter, expressed predominately in kidney and intestine, with properties of system B 0. We identified six mutations in SLC6A19 that cosegregated with disease in the predicted recessive manner, with most affected individuals being compound heterozygotes. The disease-causing mutations that we tested reduced neutral amino acid transport function in vitro. Population frequencies for the most common mutated SLC6A19 alleles are 0.007 for 517G-A and 0.001 for 718C-T. Our findings indicate that SLC6A19 is the long-sought gene that is mutated in Hartnup disorder; its identification provides the opportunity to examine the inconsistent multisystemic features of this disorder. 14 C-Amino acid uptake 14 C-Leucine uptake Figure 1 Ion and voltage dependence and substrate specificity of SLC6A19 confirm that it has the predicted profile for system B 0. (a) Leucine uptake (pmol per 15 min) in oocytes transfected with SLC6A19 cRNA in which buffer containing NaCl (Na) was modified. The transport activity of oocytes injected with water was subtracted (n ¼ 3 experiments). Expression of SLC6A19 resulted in uptake of 100 mM 14 C-leucine at 2073 pmol per 15 min, which was more than twice that detected in oocytes injected with water (net activity 871.8 pmol per 15 min). Uptake of 14 C-leucine was sodium-dependent, as replacement of sodium ions by the impermeant N-methyl-D-glucamine (NMDG) or LiCl (Li) reduced the transport activity by 92% or 85%, respectively. Leucine uptake was not chloride-dependent, as replacement of NaCl by sodium gluconate (-Cl) did not significantly alter transport activity (P40.1). Transport of amino acids by SLC6A19 was driven by membrane potential, as the addition of 50 mM KCl (+ K) to the transport buffer reduced leucine uptake by 58%. (b) Uptake (pmol per 15 min) of 14 C-labeled amino acids in oocytes transfected with SLC6A19 cRNA or controls injected with water. The specificity for neutral amino acids is shown in this experiment, which was done three times. Phenylalanine seemed to be the best substrate, followed by leucine, glutamine and alanine. Glutamate and arginine were not actively transported.
The primary structures of four bovine clathrin light chains have been determined. Light chains LCa and LCb are homologous proteins encoded by different genes. In the brain the messenger RNA from these genes undergoes differential splicing to yield proteins having centrally inserted brain-specific sequences. A potentially alpha-helical region of the clathrin light chains shows homology with intermediate filament proteins.
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