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Functional characterization of mouse sodium/glucose transporter type 3b.

Despite belonging to a family of sugar cotransporters, human sodium/glucose transporter type 3 (hSGLT3) does not transport sugar, but it depolarizes the cell in the presence of extracellular sugar, and thus it has been suggested to work as a sugar sensor. In the human genome there is one SGLT3 gene, yet in mouse there are two. In this study we cloned one of them, mouse SGLT3b (mSGLT3b) and characterized the protein. We found that mSGLT3b has low affinity for sugars, as does hSGLT3, but surprisingly, mSGLT3b transports sugar, although the sugar transport is not as tightly coupled to cations as in SGLT1. Moreover, the sugar specificity of mSGLT3b has characteristics reminiscent of both SGLT1 and hSGLT3: mSGLT3b does not respond to galactose, similar to hSGLT3, but neither does it respond to 1-deoxynojirimycin, unlike hSGLT3 but similar to SGLT1. mSGLT3b has low apparent affinities for sugar and Na(+) and, furthermore, displays pre-steady-state currents, which in SGLT1 report on conformational changes in the protein. Finally, phlorizin, the typical inhibitor of SGLT proteins, also inhibits mSGLT3b. In summary, although mSGLT3b has some characteristics that resemble SGLT1 and others that are similar to hSGLT3, its low sugar affinity and uncoupled sugar transport lead us to conclude that mSGLT3b likely functions as a physiological glucose sensor similar to hSGLT3.