X-ray structure of EmrE supports dual topology model.

EmrE, a multidrug transporter from Escherichia coli, functions as a homodimer of a small four-transmembrane protein. The membrane insertion topology of the two monomers is controversial. Although the EmrE protein was reported to have a unique orientation in the membrane, models based on electron microscopy and now defunct x-ray structures, as well as recent biochemical studies, posit an antiparallel dimer. We have now reanalyzed our x-ray data on EmrE. The corrected structures in complex with a transport substrate are highly similar to the electron microscopy structure. The first three transmembrane helices from each monomer surround the substrate binding chamber, whereas the fourth helices participate only in dimer formation. Selenomethionine markers clearly indicate an antiparallel orientation for the monomers, supporting a "dual topology" model.

High affinity humanized antibodies without making hybridomas; immunization paired with mammalian cell display and in vitro somatic hypermutation.

A method has been developed for the rapid generation of high-affinity humanized antibodies from immunized animals without the need to make conventional hybridomas. Rearranged IgH D(J) regions were amplified from the spleen and lymph tissue of mice immunized with the human complement protein C5, fused with a limited repertoire of human germline heavy chain V-genes to form intact humanized heavy chains, and paired with a human light chain library. Completed heavy and light chains were assembled for mammalian cell surface display and transfected into HEK 293 cells co-expressing activation-induced cytidine deaminase (AID). Numerous clones were isolated by fluorescence-activated cell sorting, and affinity maturation, initiated by AID, resulted in the rapid evolution of high affinity, functional antibodies. This approach enables the efficient sampling of an immune repertoire and the direct selection and maturation of high-affinity, humanized IgGs.

X-ray structure of the EmrE multidrug transporter in complex with a substrate.

EmrE is a prototype of the Small Multidrug Resistance family of efflux transporters and actively expels positively charged hydrophobic drugs across the inner membrane of Escherichia coli. Here, we report the x-ray crystal structure, at 3.7 angstrom resolution, of one conformational state of the EmrE transporter in complex with a translocation substrate, tetraphenylphosphonium. Two EmrE polypeptides form a homodimeric transporter that binds substrate at the dimerization interface. The two subunits have opposite orientations in the membrane and adopt slightly different folds, forming an asymmetric antiparallel dimer. This unusual architecture likely confers unidirectionality to transport by creating an asymmetric substrate translocation pathway. On the basis of available structural data, we propose a model for the proton-dependent drug efflux mechanism of EmrE.