title: Ribosome assembly factors prevent premature translation initiation by 40S assembly intermediates.
abstract: Ribosome assembly in eukaryotes requires approximately 200 essential assembly factors (AFs) and occurs through ordered events that initiate in the nucleolus and culminate in the cytoplasm. Here, we present the electron cryo-microscopy (cryo-EM) structure of a late cytoplasmic 40S ribosome assembly intermediate from Saccharomyces cerevisiae at 18 angstrom resolution. We obtained cryo-EM reconstructions of preribosomal complexes lacking individual components to define the positions of all seven AFs bound to this intermediate. These late-binding AFs are positioned to prevent each step in the translation initiation pathway. Together, they obstruct the binding sites for initiation factors, prevent the opening of the messenger RNA channel, block 60S subunit joining, and disrupt the decoding site. These redundant mechanisms probably ensure that pre-40S particles do not enter the translation pathway, which would result in their rapid degradation.
authors.affiliation: Chemical Biology Doctoral Program, University of Michigan, Ann Arbor, MI 48109, USA.
authors.forename: Bethany S
authors.initials: BS
authors.lastname: Strunk
authors.forename: Cherisse R
authors.initials: CR
authors.lastname: Loucks
authors.forename: Min
authors.initials: M
authors.lastname: Su
authors.forename: Harish
authors.initials: H
authors.lastname: Vashisth
authors.forename: Shanshan
authors.initials: S
authors.lastname: Cheng
authors.forename: Justin
authors.initials: J
authors.lastname: Schilling
authors.forename: Charles L
authors.initials: CL
authors.lastname: Brooks
authors.forename: Katrin
authors.initials: K
authors.lastname: Karbstein
authors.forename: Georgios
authors.initials: G
authors.lastname: Skiniotis
mesh_terms: D001665:Binding Sites; D020285:Cryoelectron Microscopy; D015977:Eukaryotic Initiation Factor-1 / Q000737:chemistry / Q000378:metabolism; D039621:Eukaryotic Initiation Factor-3 / Q000737:chemistry / Q000378:metabolism; D007091:Image Processing, Computer-Assisted; D008780:Methyltransferases / Q000737:chemistry / Q000378:metabolism; D008958:Models, Molecular; D009687:Nuclear Proteins / Q000737:chemistry / Q000378:metabolism; D010442:Peptide Chain Initiation, Translational; D017346:Protein Serine-Threonine Kinases / Q000737:chemistry / Q000378:metabolism; D012331:RNA, Fungal / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D012269:Ribosomal Proteins / Q000737:chemistry / Q000378:metabolism; D054682:Ribosome Subunits, Small, Eukaryotic / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure; D012441:Saccharomyces cerevisiae / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D015977:Eukaryotic Initiation Factor-1; D039621:Eukaryotic Initiation Factor-3; C501335:LTV1 protein, S cerevisiae; C405962:NOB1 protein, S cerevisiae; D009687:Nuclear Proteins; D012331:RNA, Fungal; D012333:RNA, Messenger; D012269:Ribosomal Proteins; D029701:Saccharomyces cerevisiae Proteins; C026022:eukaryotic peptide initiation factor-1A; C088929:DIM1 protein, S cerevisiae; D008780:Methyltransferases; D017346:Protein Serine-Threonine Kinases; C473025:Rio2 protein, S cerevisiae