Functional characterization of CsGTP5, a sodium-independent glucose transporter-related protein in <i>Clonorchis sinensis</i>

Glucose acquisition is essential for the long-term survival of adult Clonorchis sinensis within the host bile duct. Although several glucose transporter-related proteins have been identified in C. sinensis, the molecular and functional properties of facilitative glucose transport proteins, including the protein designated as glucose transporter protein 5 (CsGTP5) in this study, have not been characterized. In this study, CsGTP5 was cloned from adult C. sinensis and functionally analyzed using the Xenopus laevis oocyte expression system. The CsGTP5 open reading frame comprised 1,575 bp and encoded a 524-amino-acid protein with 12 predicted transmembrane domains, consistent with the topology of facilitative glucose transporter-like proteins. CsGTP5-expressing oocytes showed significantly increased uptake of [3 H] deoxy-D-glucose compared with water-injected control oocytes, whereas no significant uptake was observed for other tested substrates, including arginine, α-ketoglutarate, p-aminohippurate, taurocholate, and tetraethylammonium. CsGTP5-mediated [3 H] deoxy-D-glucose uptake increased in a time-dependent manner and was not affected by replacement of extracellular Na+ with Li+ or choline, indicating sodium-independent transport. Uptake was saturable, with an apparent Km of 4.0 mM and a Vmax of 300.0 pmol/oocyte/h. Competition assays showed that deoxy-D-glucose and glucose strongly inhibited CsGTP5-mediated uptake, whereas other monosaccharides had little effect. Molecular docking analysis further supported the predicted glucose-recognition capacity of CsGTP5. These findings demonstrate that CsGTP5 mediates sodium-independent deoxy-D-glucose uptake in a heterologous oocyte expression system and provide a basis for further investigation of its physiological role in C. sinensis.