@article{THESIS,
      recid = {10150},
      author = {Zhong, Frank Wang},
      title = {Defining Assembly of a Translocon for Multipass Membrane  Protein Biogenesis},
      publisher = {University of Chicago},
      school = {Ph.D.},
      address = {2023-12},
      number = {THESIS},
      pages = {129},
      abstract = {The cellular proteome is comprised of both soluble  proteins and membrane proteins.  Membrane proteins make up  30% of the human proteome and defects in their biogenesis  results  in a wide range of diseases. While biogenesis of  membrane proteins with a singular  transmembrane domain  (TMD) is relatively well understood, the biogenesis of  membrane  proteins with multiple TMDs (“multipass”) is less  clear. The TMDs of multipass membrane  proteins are  diverse, with varying hydrophobicities, and require folding  within the lipid bilayer.  To overcome this set of  challenges, the Sec61 translocon has been shown to assemble  various  complexes during protein synthesis. Here, we show  that for multipass membrane proteins, a  Sec61 translocon  specific for this class of membrane proteins exists and is  assembled in response  to defined signals. Furthermore, we  show that two different novel accessory factors associate   with the Sec61 translocon and are involved in protein  biogenesis.   We first define the existence of a “multipass  translocon.” The multipass translocon is  distinguished by  three components that selectively bind the ribosome–Sec61  complex during  multipass protein synthesis: the GET- and  EMC-like (GEL), protein associated with translocon  (PAT)  and back of Sec61 (BOS) complexes. We show that assembly of  the multipass translocon  is in response to defined signals  and depletion of multipass translocon components results in   multipass membrane protein biogenesis and topogenesis  defects both in vivo and in vitro. These  results establish  the mechanism by which nascent multipass proteins  selectively recruit the  multipass translocon to facilitate  their biogenesis.   In addition to this work, we  characterize the role of two translocon accessory factors,   RAMP4 and FKBP11, and their role in protein biogenesis. For  both proteins, we demonstrate      xi  that they are  involved in the synthesis of proteins with long lumenal  loops. Specifically, for  FKBP11, RNA sequencing of  ribosome-translocon complexes implicates a bias for  proteins that  have long translocated lumenal domains.  Analysis of insertion intermediates shows dynamics of   FKBP11 association with the Sec61 translocon, with  recruitment being concomitant with the  oligosaccharyl  transferase complex (OST). Depletion of FKBP11 further  leads to membrane  protein instability at the cellular  level. In summation, this work defines the ER translocon as  a  dynamic assembly whose subunit composition adjusts  co-translationally to accommodate the  biosynthetic needs  of its diverse range of substrates, from single-pass to  multi-pass.},
      url = {http://knowledge.uchicago.edu/record/10150},
      doi = {https://doi.org/10.6082/uchicago.10150},
}