title: Translation factors promote the formation of two states of the closed-loop mRNP.
abstract: Efficient translation initiation and optimal stability of most eukaryotic messenger RNAs depends on the formation of a closed-loop structure and the resulting synergistic interplay between the 5' m(7)G cap and the 3' poly(A) tail. Evidence of eIF4G and Pab1 interaction supports the notion of a closed-loop mRNP, but the mechanistic events that lead to its formation and maintenance are still unknown. Here we use toeprinting and polysome profiling assays to delineate ribosome positioning at initiator AUG codons and ribosome-mRNA association, respectively, and find that two distinct stable (resistant to cap analogue) closed-loop structures are formed during initiation in yeast cell-free extracts. The integrity of both forms requires the mRNA cap and poly(A) tail, as well as eIF4E, eIF4G, Pab1 and eIF3, and is dependent on the length of both the mRNA and the poly(A) tail. Formation of the first structure requires the 48S ribosomal complex, whereas the second requires an 80S ribosome and the termination factors eRF3/Sup35 and eRF1/Sup45. The involvement of the termination factors is independent of a termination event.
authors.affiliation: Department of Molecular Genetics and Microbiology, University of Massachusetts Medical School, Worcester, Massachusetts 01655-0122, USA.
authors.forename: Nadia
authors.initials: N
authors.lastname: Amrani
authors.forename: Shubhendu
authors.initials: S
authors.lastname: Ghosh
authors.forename: David A
authors.initials: DA
authors.lastname: Mangus
authors.forename: Allan
authors.initials: A
authors.lastname: Jacobson
mesh_terms: D000818:Animals; D001483:Base Sequence; D018387:Codon, Initiator / Q000235:genetics; D003513:Cycloheximide / Q000494:pharmacology; D039603:Eukaryotic Initiation Factor-4G / Q000378:metabolism; D010454:Peptide Termination Factors / Q000235:genetics / Q000378:metabolism; D039102:Poly(A)-Binding Proteins / Q000378:metabolism; D011132:Polyribosomes / Q000235:genetics / Q000378:metabolism; D011328:Prions / Q000378:metabolism; D014176:Protein Biosynthesis; D012315:RNA Caps / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D012261:Ribonucleoproteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D012269:Ribosomal Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D012270:Ribosomes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics* / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism
chemical_list: D018387:Codon, Initiator; D039603:Eukaryotic Initiation Factor-4G; D010454:Peptide Termination Factors; D039102:Poly(A)-Binding Proteins; D011328:Prions; D012315:RNA Caps; D012333:RNA, Messenger; D012261:Ribonucleoproteins; D012269:Ribosomal Proteins; C068068:SUP35 protein, S cerevisiae; C080775:SUP45 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; C026452:messenger ribonucleoprotein; C450100:pab1 protein, S cerevisiae; C087971:peptide-chain-release factor 3; D003513:Cycloheximide