The goal of this research is to understand the mechanism of co-translational translocation of nascent secretory and membrane proteins in molecular detail. This process occurs at the plasma membrane in bacteria and at the ER membrane in eukaryotes. Translocation is mediated by the SecY or Sec61 channel bound to its cognate ribosome. In eukaryotes, complementary studies of the translocon associated protein complex (TRAP) will be continued and the role of the ribosome-Bag6 complex in the insertion pathway for tail-anchored membrane proteins will be investigated.
In Aim 1, four structures will be determined of E. coli ribosome-SecY complexes at various stages of translocation of a secretory protein. We will use state-of-the-art cryo-EM images taken on a direct electron detector to provide high resolution 3D maps of the complexes. This will include a ground state structure without a nascent chain and structures of secretory complexes with nascent chains at early and late stages of translocation. In these studies, active 70S-SecY complexes are formed in cells and stalled nascent chains are crosslinked within the channel to trap the translocation intermediates.
This Aim will reveal conformational changes of the SecY complex and the disposition of the nascent chain, as the lateral gate opens to allow a signal sequence helix to move into the surrounding bilayer and be processed by signal sequence peptidase. This cleavage step leaves a SecY channel with a single strand of the translocating nascent chain threaded across the pore.
In Aim 2, the goal is to obtain a crystal structure of the alpha/beta lumenal domain of TRAP to test our hypothesis that the alpha subunit may act like a chaperone that transiently binds to extended nascent chains as they exit from the Sec61 channel. This may bias translocation in the forward direction.
In Aim 3, the structure of a human Bag6-TRC35-Ubl4A complex will be determined, as it interacts with 80S ribosomes that contain a transmembrane (TM) domain in the exit tunnel. This is the first step of the insertion pathway for tail anchored membrane proteins, in which the TM helix is protected from aggregation by the Bag6 complex. The TM helix is subsequently delivered to the TRC40 ATPase dimer, which transfers its substrate to the Get1-Get2 complex for insertion into the ER membrane.

Public Health Relevance

Ribosomes, protein conducting channel complexes and their protein co-factors carry out essential steps in protein synthesis and their targeting within growing cells. These components play critical roles in the biogenesis of secretory proteins, such as peptide hormones, growth factors and apolipoproteins, as well as all membrane proteins. Aberrant function of these complexes is a causative factor in the etiology of human disease. For example, mis-translocation of human prion proteins at the ER has been implicated in neurodegeneration.

Agency
National Institute of Health (NIH)
Institute
National Institute of General Medical Sciences (NIGMS)
Type
Research Project (R01)
Project #
5R01GM045377-21
Application #
9133388
Study Section
Macromolecular Structure and Function C Study Section (MSFC)
Program Officer
Flicker, Paula F
Project Start
1991-01-01
Project End
2019-06-30
Budget Start
2016-07-01
Budget End
2017-06-30
Support Year
21
Fiscal Year
2016
Total Cost
Indirect Cost
Name
Boston University
Department
Physiology
Type
Schools of Medicine
DUNS #
604483045
City
Boston
State
MA
Country
United States
Zip Code
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