Sugar is a vital molecule required for cell viability and homeostasis. Sugar is important for metabolic energy, energy storage, signaling, structure and osmolyte regulation. Transport of sugar represents an important physiological process. Specific membrane transporter families have evolved to mediate the transport of sugar across biological membranes. In this thesis, we describe our work leading to a better mechanistic understanding of two sugar transporter families, namely glucose (GLUT) transporters and nucleotide-sugar (NST) transporters.

Members of GLUT transporters, belonging to the Solute Carrier (SLC2) family, are involved in the uptake of various monosaccharides across the cellular membranes. Activity of different NSTs, belonging to the (SLC35) family, is crucial for the process of glycosylation by mediating the translocation of activated sugars from the cytoplasm into the lumen of either Golgi and/or ER organelles. GLUTs and NSTs families carry out transport processes fundamental to human physiology and pathophysiology. Despite the profound importance of GLUTs and NSTs in human health, comprehensive understanding of their architecture and mechanistic features with respect to determinants of substrate binding and allosteric coupling at the molecular level has remained elusive.

In this thesis, we address key functional and structural properties of GLUT and NST mediated sugar transport. We combine crystal structures with robust binding and transport assays as well as computational approaches. The role of lipids in fine-tuning the activity of transporters is also exemplified by demonstrating the effect of lipid composition in the transport activity of GLUTs using in-vitro proteoliposome assays. Our work has not only enhanced the current understanding of GLUT and NST function, but also developed themes and methods that are likely relevant to many types of small molecule transporters.