Dr. Kakoli Mitra
HHMI at Wadsworth Center
Albany, NY

Monday, December 11, 2006
3:00 pm (CT)
3269 Beckman Institute

Abstract

The second half of the central dogma of biology states that the information encoded in RNA is translated into protein. Translation of messenger RNA (mRNA) into a polypeptide chain is a multi-component process coordinated by the ribosome, a ribonucleoprotein complex. Electron microscopy (EM) not only played a key role in the discovery of the ribosome in 1955, but with the development of three-dimensional reconstruction techniques has provided detailed snapshots of the ribosome in different conformations at distinct stages of the translation process. With the advent of improved fitting methods it is now possible to interpret EM maps at the quasi-atomic level using X-ray crystallographic models. The recent application of this approach to an EM structure of a ribosome stalled by a nascent SecM polypeptide resulted in a new hypothesis: signals are transduced over large distances within the ribosome through a cascade of rearrangements of ribosomal RNA. Beyond the central dogma, the cell requires that a large number of proteins translocate across or integrate into a membrane. Translocation/integration occurs via the protein-conducting channel (PCC) – an integral membrane protein complex – either co- or post-translationally. Recently, an EM structure was obtained of a polypeptide-translocating ribosome bound to a PCC and interpreted at the quasi-atomic level using flexible fitting methods. The interpretation suggests that the PCC consists of a front-to-front dimer, in which one protomer is dedicated to protein translocation across, and the other protomer to protein integration into, the membrane. The EM structure and further analysis form the basis of a new integrated framework for protein translocation, suggesting that analogous mechanisms are at work at the PCC in both the co- and post-translational pathways. The future application of fluorescence spectroscopy and EM to macromolecular complexes composed of combinations of the ribosome, the PCC, and other co-factors of translation/translocation, will test these newly emerging hypotheses and deepen our understanding of the mechanisms underlying these processes.