The human IGF1R IRES likely operates through a Shine-Dalgarno-like interaction with the G961 loop (E-site) of the 18S rRNA and is kinetically modulated by a naturally polymorphic polyU loop.
IGF1R is a proto-oncogene with potent mitogenic and antiapoptotic activities, and its expression must be tightly regulated to maintain normal cellular and tissue homeostasis. We previously demonstrated that translation of the human IGF1R mRNA is controlled by an internal ribosome entry site (IRES), and delimited the core functional IRES to a 90-nucleotide segment of the 5'-untranslated region positioned immediately upstream of the initiation codon. Here we have analyzed the sequence elements that contribute to the function of the core IRES. The Stem2/Loop2 sequence of the IRES exhibits near-perfect Watson-Crick complementarity to the G961 loop (helix 23b) of the 18S rRNA, which is positioned within the E-site on the platform of the 40S ribosomal subunit. Mutations that disrupt this complementarity have a negative impact on regulatory protein binding and dramatically decrease IRES activity, suggesting that the IGF1R IRES may recruit the 40S ribosome by a eukaryotic equivalent of the Shine-Dalgarno (mRNA-rRNA base-pairing) interaction. The homopolymeric Loop3 sequence of the IRES modulates accessibility and limits the rate of translation initiation mediated through the IRES. Two functionally distinct allelic forms of the Loop3 poly(U)-tract are prevalent in the human population, and it is conceivable that germ-line or somatic variations in this sequence could predispose individuals to development of malignancy, or provide a selectable growth advantage for tumor cells.
Meng
Zheng
Z
Department of Biochemistry and Molecular Genetics, University of Alabama at Birmingham, Birmingham, Alabama 35294, USA.
Jackson
Nateka L
NL
Shcherbakov
Oleg D
OD
Choi
Hyoungsoo
H
Blume
Scott W
SW
D020121:5' Untranslated Regions; D000483:Alleles; D001483:Base Sequence; D045744:Cell Line, Tumor; D017931:DNA Primers; D005786:Gene Expression Regulation; D017930:Genes, Reporter; D006801:Humans; D007700:Kinetics; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D016133:Polymerase Chain Reaction; D011110:Polymorphism, Genetic; D000090063:Proto-Oncogene Mas; D012337:RNA, Ribosomal, 18S / Q000235:genetics*; D017526:Receptor, IGF Type 1 / Q000502:physiology*
D020121:5' Untranslated Regions; D017931:DNA Primers; C000715470:MAS1 protein, human; D000090063:Proto-Oncogene Mas; D012337:RNA, Ribosomal, 18S; D017526:Receptor, IGF Type 1