Gira Bhabha, PhD

From atoms to cells to organisms, life is in constant motion. All cells face the challenge of transporting molecules from one location to another within the cell, and this is essential for the cell to function normally. Proteins that are responsible for proper transport range from simple globular proteins to large multi-protein complexes. We use a hybrid approach to study the structural mechanisms of how macromolecular protein machines are so spectacularly coordinated in normal cells, and what goes wrong when they break.

We are interested in understanding how structure, function and dynamics of proteins are coordinated to function in biological systems. We use structural biology techniques such as Cryo EM and X-ray crystallography, in combination with biochemical and functional assays to study complex   macromolecular machines.

HOW DO MICROSPORIDIA USE A HARPOON-LIKE INVASION MACHINERY TO INFECT HOST CELLS?
Microsporidia are intracellular eukaryotic parasites that infect a wide range of hosts, from insects to vertebrates.  In order to gain entry into a target cell, microsporidia employ a remarkable harpoon-like apparatus called the polar tube. While initially coiled neatly within the spore of the parasite, infection of a new cell begins with the rapid extrusion of the polar tube from the spore, which pierces the membrane of a nearby host cell and anchors the spore in place. The polar tube is then thought to act as a conduit for the transfer of the infectious sporoplasm into the target cell, where replication can begin. The molecular and structural underpinnings of the invasion process remain unclear and are ripe for exploration using hybrid structural biology tools such as cryo electron microscopy and X-ray crystallography.  We are aiming to address fundamental questions about the polar tube and how it drives invasion of host cells.

HOW DO MCE PROTEINS FUNCTION TO MAINTAIN THE INTEGRITY OF THE BACTERIAL OUTER MEMBRANE?
All cells face the challenge of transporting hydrophobic lipids from one membrane to another through an intervening aqueous environment.  Eukaryotes solve this trafficking problem using small transport vesicles that shuttle between membrane compartments. Many bacteria face a similar problem of transporting lipids between the inner and outer membrane through an aqueous periplasm, but lack vesicular transport systems. Previous work has uncovered how lipopolysaccharide (LPS), one major component of the outer membrane, is trafficked. However, a mechanistic understanding of how phospholipids are trafficked between the inner and outer membranes remains an important unsolved question in prokaryotic  biology. We are working on obtaining a structural and mechanistic understanding of how the MCE (Mammalian Cell Entry) family of proteins facilitate lipid transport across membranes.

UNDERSTANDING THE MECHANISM OF MOTILITY IN THE MOTOR PROTEIN DYNEIN
Dynein is a motor protein that walks towards the minus end of microtubule polymers in most eukaryotic cells, and can broadly be divided into two kinds: Cytoplasmic and Axonemal. Cytoplasmic dynein transport cargos within cells, while axonemal dynein is responsible for sliding microtubules of the axoneme in cilia to generate highly patterned ciliary beating. Cytoplasmic dynein is a large, awkwardly shaped, homo-dimeric motor protein that belongs to the AAA family of proteins. Like other motor proteins, such as kinesin, dynein can be thought to have two main parts: a motor domain, which is responsible for movement along microtubules, and a cargo-binding domain, which is responsible for attaching the motor to various cargoes. In contrast to the well studied motor domains of kinesin or myosin, the motor domain of dynein is remarkably complex, consisting of a AAA ring at one end, and a small microtubule-binding domain at the other. The two are separated by a long coiled-coil, making the site of chemistry spatially distant from the tiny microtubule-binding domain, which is the business end of the molecule. We aim to understand the coordination of events in a dynein dimer that allows it to move along a microtubule in the correct direction. While many elements of cytoplasmic dynein are also conserved in axonemal dyneins, there are some key differences between the two, and we also aim to explore this further, with the goal of understanding how axonemal dyneins drive movement in motile cilia.