Structure of the Membrane Protein FhaC: A Member of the Omp85-TpsB Transporter Superfamily

Nano-scale characterization of the dynamics of the chloroplast Toc translocon

Translocons are macromolecular nano-scale machines that facilitate the selective translocation of proteins across membranes. Although common in function, different translocons have evolved diverse molecular mechanisms for protein translocation. Subcellular organelles of endosymbiotic origin such as the chloroplast and mitochondria had to evolve/acquire translocons capable of importing proteins whose genes were transferred to the host genome. These gene products are expressed on cytosolic ribosomes as precursor proteins and targeted back to the organelle by an N-terminal extension called the transit peptide or presequence. In chloroplasts the transit peptide is specifically recognized by the Translocon of the Outer Chloroplast membrane (Toc) which is composed of receptor GTPases that potentially function as gate-like switches, where GTP binding and hydrolysis somehow facilitate preprotein binding and translocation. Compared to other translocons, the dynamics of the Toc translocon are probably more complex and certainly less understood. We have developed biochemical/biophysical, imaging, and computational techniques to probe the dynamics of the Toc translocon at the nanoscale. In this chapter we provide detailed protocols for kinetic and binding analysis of precursor interactions in organeller, measurement of the activity and nucleotide binding of the Toc GTPases, native electrophoretic analysis of the assembly/organization of the Toc complex, visualization of the distribution and mobility of Toc apparatus on the surface of chloroplasts, and conclude with the identification and molecular modeling Toc75 POTRA domains. With these new methodologies we discuss future directions of the field.

First glimpse of the crystal structure of YaeT's POTRA domains

The Omp85/YaeT family of proteins, which are conserved from bacteria to human, catalyzes insertion and assembly of proteins in the outer membrane. The structure consists of a transmembrane beta-barrel domain and a soluble polypeptide-transport-associated (POTRA) domain. The POTRA domain is critical for substrate recognition and perhaps substrate folding, while the beta-barrel domain assists in membrane insertion. The resolution of the crystal structure of the POTRA domain of the Escherichia coli YaeT protein provides a possible molecular mechanism by which the diverse group of substrates is recognized. Knowledge gained from the crystal structure may also spur the development of a novel class of chemotherapeutic inhibitors.