Cryo-EM structure of the potassium-chloride cotransporter KCC4 in lipid nanodiscs.

Cation-chloride-cotransporters (CCCs) catalyze transport of Cl- with K+ and/or Na+across cellular membranes. CCCs play roles in cellular volume regulation, neural development and function, audition, regulation of blood pressure, and renal function. CCCs are targets of clinically important drugs including loop diuretics and their disruption has been implicated in pathophysiology including epilepsy, hearing loss, and the genetic disorders Andermann, Gitelman, and Bartter syndromes. Here we present the structure of a CCC, the Mus musculus K+-Cl- cotransporter (KCC) KCC4, in lipid nanodiscs determined by cryo-EM. The structure, captured in an inside-open conformation, reveals the architecture of KCCs including an extracellular domain poised to regulate transport activity through an outer gate. We identify binding sites for substrate K+ and Cl- ions, demonstrate the importance of key coordinating residues for transporter activity, and provide a structural explanation for varied substrate specificity and ion transport ratio among CCCs. These results provide mechanistic insight into the function and regulation of a physiologically important transporter family

Structural elucidation of a common architecture of the nuclear pore complex and COPIl vesicle coats

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Biology, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 156-169).Nuclear pore complexes (NPCs) are massive protein assemblies that perforate the nuclear envelope and form the exclusive passageway for nucleocytoplasmic transport. NPCs play critical roles in molecular transport and a myriad of other cellular processes. Elucidation of the structure of the NPC is thus expected to provide important insight into cell biology. In this thesis, I investigate the structure of a key subcomplex of the NPC and discuss the evolutionary relationship between the NPC and COPIl vesicle coats it illustrates. The NPC is a modular assembly, with a stable structural scaffold supporting dynamically attached components. The structural scaffold is constructed from multiple copies of the Y-shaped complex and the Nic96 complex. We solved the crystal structure of the heterodimeric Nup85-Sehl module that forms a short arm in the Y complex. Nup85 is found to contain a conserved fold, the ancestral coatomer element 1 (ACE1), also present in three other components of the NPC and in the COPI vesicle coat, providing structural evidence of coevolution from a common ancestor. Sec3l ACE1 units interact to form edge elements in the COPI lattice. Using structural knowledge of this edge element, we identified corresponding interactions between ACE1 proteins Nup84 and Nup145C in the NPC. We solved the crystal structure of the heterotrimeric Nup84-Nupl 45C-Secl 3 module that forms the top of the long arm in the Y complex. The heterotypic ACE1 interaction of Nup145C and Nup84 is analogous to the homotypic Sec31 edge element interaction in the COPIl coat. From these and other structures, we assemble a near complete structural model of the Y complex. Further, based on the demonstrated relationship with the COPIl coatomer lattice, we propose a lattice model for the entire NPC scaffold. The common architectural principles of the edge elements in the NPC and COPI lead us to predict that Y complexes will be arranged as struts in the NPC lattice. In this manner, Nup84-Nup145C edge elements are arranged parallel to the transport axis to stabilize the positively curved nuclear envelope. From a lattice model of the NPC follow hypotheses for how other components are integrated into and function within the NPC.by Stephen Graf Brohawn.Ph.D