Integral membrane transport proteins are essential for the transport of a wide variety of substrates such as ions, drugs and metabolites across the membranes of microorganisms. They are found in diverse locations such as outer and inner membranes of archaeal, prokaryotic and eukaryotic cells, as well as the outer layers of the membrane-bound organelles, including chloroplasts, endoplasmic reticulum and mitochondria. These proteins are capable of facilitating the transport of substrates both from outside the cell to inside, and vice versa, and they can be very specific, thereby playing a significant role in substrate selection. Integral membrane transport proteins also play important roles in regulation of transport, defense against drugs and antibiotics, and hemostasis. Thus, understanding the structural framework and detailed functional mechanisms of these proteins will contribute to our knowledge of biological processes.

In this study, we have investigated the structure, function and regulation of three different types of integral membrane transport proteins. Chapter 2 focuses on elucidating the structure and function of the transcriptional regulator Rv0678 of Mycobacterium tuberculosis, which negatively regulates the expression of the MmpS4-MmpL4 system, which plays a key role in the biosynthesis and transport of lipids from the cytoplasm towards the periplasm. Lipids play an important role in cell wall remodeling and permeability, thereby functioning in bacterial defense against antibiotics. We crystallized Rv0678 and identified the ligand bound to the protein as 2-stearoylglycerol. We used the structure and functional studies to elucidate a possible DNA binding mechanism for the protein. Chapter 3 discusses the crystal structure of the outer membrane channel, CmeC, of CmeABC efflux system of Campylobacter jejuni. CmeABC is a multidrug efflux system that pumps out quinolones and bile acid derivatives. The crystal structure revealed the structural framework of the channel and captured it in closed conformation. Chapter 4 focuses on understanding the structural basis of nitrite transport into the chloroplast in Chlamydomonas reinhardtii. Formate-nitrite transporter family proteins, NAR1.1 and NAR1.5, were crystallized using hanging drop vapor diffusion. Stopped flow light scattering experiments were carried out to identify nitrite as the substrate for both proteins. Site-directed mutagenesis revealed key residues that form the channel and allowed us to propose a possible nitrite transport mechanism.