title: Hrr25-dependent phosphorylation state regulates organization of the pre-40S subunit.
abstract: The formation of eukaryotic ribosomes is a multistep process that takes place successively in the nucleolar, nucleoplasmic and cytoplasmic compartments. Along this pathway, multiple pre-ribosomal particles are generated, which transiently associate with numerous non-ribosomal factors before mature 60S and 40S subunits are formed. However, most mechanistic details of ribosome biogenesis are still unknown. Here we identify a maturation step of the yeast pre-40S subunit that is regulated by the protein kinase Hrr25 and involves ribosomal protein Rps3. A high salt concentration releases Rps3 from isolated pre-40S particles but not from mature 40S subunits. Electron microscopy indicates that pre-40S particles lack a structural landmark present in mature 40S subunits, the 'beak'. The beak is formed by the protrusion of 18S ribosomal RNA helix 33, which is in close vicinity to Rps3. Two protein kinases Hrr25 and Rio2 are associated with pre-40S particles. Hrr25 phosphorylates Rps3 and the 40S synthesis factor Enp1. Phosphorylated Rsp3 and Enp1 readily dissociate from the pre-ribosome, whereas subsequent dephosphorylation induces formation of the beak structure and salt-resistant integration of Rps3 into the 40S subunit. In vivo depletion of Hrr25 inhibits growth and leads to the accumulation of immature 40S subunits that contain unstably bound Rps3. We conclude that the kinase activity of Hrr25 regulates the maturation of 40S ribosomal subunits.
authors.affiliation: Biochemie-Zentrum der Universität Heidelberg, Im Neuenheimer Feld 328, 69120 Heidelberg, Germany.
authors.forename: Thorsten
authors.initials: T
authors.lastname: Schäfer
authors.forename: Bohumil
authors.initials: B
authors.lastname: Maco
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Petfalski
authors.forename: David
authors.initials: D
authors.lastname: Tollervey
authors.forename: Bettina
authors.initials: B
authors.lastname: Böttcher
authors.forename: Ueli
authors.initials: U
authors.lastname: Aebi
authors.forename: Ed
authors.initials: E
authors.lastname: Hurt
mesh_terms: D047389:Casein Kinase I / Q000737:chemistry* / Q000378:metabolism*; D008958:Models, Molecular; D009687:Nuclear Proteins / Q000378:metabolism; D010766:Phosphorylation; D011487:Protein Conformation; D017346:Protein Serine-Threonine Kinases; D021122:Protein Subunits / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D012269:Ribosomal Proteins / Q000378:metabolism; D012270:Ribosomes / Q000737:chemistry* / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry* / Q000378:metabolism*
chemical_list: C104527:ENP1 protein, S cerevisiae; C501335:LTV1 protein, S cerevisiae; D009687:Nuclear Proteins; D021122:Protein Subunits; C501334:RPS3 protein, S cerevisiae; D012269:Ribosomal Proteins; D029701:Saccharomyces cerevisiae Proteins; D047389:Casein Kinase I; C070775:HRR25 protein, S cerevisiae; D017346:Protein Serine-Threonine Kinases; C473025:Rio2 protein, S cerevisiae