PT  - JOURNAL ARTICLE
AU  - Jiujiu Huang
AU  - Nan Li
AU  - Weifang Hong
AU  - Kai Zhan
AU  - Xuan Yu
AU  - Hong Huang
AU  - Mei Hong
TI  - Conserved Tryptophan Residues within Putative Transmembrane Domain 6 Affect Transport Function of Organic Anion Transporting Polypeptide 1B1
AID  - 10.1124/mol.113.085977
DP  - 2013 Jul 15
TA  - Molecular Pharmacology
PG  - mol.113.085977
4099  - http://molpharm.aspetjournals.org/content/early/2013/07/15/mol.113.085977.short
4100  - http://molpharm.aspetjournals.org/content/early/2013/07/15/mol.113.085977.full
AB  - The organic anion-transporting polypeptides (OATPs, gene symbol SLCO) are a family of transporters that play important roles in the absorption, distribution, metabolism and excretion of various drugs. Although substrate specificity of transporter proteins is under extensive study, the underlying mechanisms for substrate binding and/or recognition remain largely unknown. Transmembrane domain 6 (TM6) is a relatively conserved region within OATP family members and several amino acid residues on its extracellular half are part of the OATP family signature sequence D-X-RW-(I,V)-GAWWX-G-(F,L)-L. In the present study, two adjacent tryptophan residues (Trp258 and Trp259) within TM6 were identified as critical amino acids for the transport function of OATP1B1. Kinetic studies showed that substitution of Trp258 with alanine resulted in monophasic kinetics for estrone-3-sulfate uptake, with a significantly higher Km value (Km=12.0±2.8μM) compared to the high affinity component of wild-type OATP1B1 (Km=0.38±0.06μM). On the other hand, W259A retained the biphasic characteristic of the transporter. Km values of the high and low affinity components for estrone-3-sulfate of W259A are 1.93±0.76μM and 30.8±4.4μM, respectively. Further studies revealed that W258A retained transport function of another prototypic substrate taurocholate; while W259A displayed a dramatically reduced uptake of the substrate and exhibited an eight-fold increase in the Km value compared with that of the wild-type and W258A. Our results suggested that Trp258 and Trp259 may play different roles for the uptake of different substrates by OATP1B1.