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Strength of splice sites some sites are weak and can

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Unformatted text preview: sed under certain conditions Recognition and use of weak splice sites is due to RNA sequences including exonic splicing enhancers Exonic splicing enhancers serve as binding sites for regulatory proteins If a particular regulatory protein is present and active it can bind to splicing enhancer and can recruit splicing machinery = inclusion of exon Alternative splicing is hard to study must extract full length mRNA from different tissues at different points in development 70% of human genes may be alternatively spliced Give the potential to create large number of polypeptides from a limited number of genes even if no amino acid change can effect 5 or 3 UTRs (regulation of translation) or NMD RNA Editing o o o Specific nucleotides can be converted to other nucleotides through mRNA editing Can create new splice sites, generate stop codons or leaf to amino acid substitutions It is important in the nervous system, where messeges need to have A converted to I (inosine) to generate a glutamate receptor Translational Level Control o o Translational level control occurs via interactions of specific mRNAs and proteins in the cytoplasm localization of mRNA to certain sites, whether or not mRNA is translated or not, how often the mRNA is translated and mRNA half life Regulatory proteins act on untranslated regions (UTRs) both 5 and 3 ends Cytoplasmic Localization of mRNAs o o o o o o Localization of mRNAs happens in all polarized cells Cytoplasmic localization of mRNAs is determined by their 3 UTRs It is mediated byt RNA binding proteins that recognize localization sequences (zip codons) Microtubules and associate motor proteins transport mRNA-containing particles to particular locations Microfilaments anchor mRNA once there How do the cells know what turns into what? Example: fluit fry anterior and posterior bicoid mRNA found in anterior side and oskar found in posterior differenences in mRNA localization occurs very early in development Control of mRNA Translation o mRNAs that were synthesized at a previous time are stored in the cytoplasm in an inactive state 6 o o o typically have shorted polyA tails eventual translation involves removal of bound inhibitory proteins, increase in length of polyA tail example: mRNA in an unfertilized egg is in inactive state in the cytoplasm - Translational Activation of mRNAs following fertilization of an Xenopus egg o o o o phosphorylation of CPEB(protein that binds to sequences in the 3 UTR of mRNA) displaces Maskin (inhibitory protein) and recruits CPSF CPSF recruits polyA polymerase POlyA tail recruits PABP PABP recruits eIF4G (initiation factor required for translation) Control of mRNA translation continued o o o o Can regulate the rate of translation of mRNAs in response to certain stimuli It can acto globally affect translation of all mRNAs Other mechanisms influence the rate of translation of specific mRNAs proteins that recognize specific elements in the UTRs of those mRNAs Example: when human cell subjected to certain stressful stimuli: protein kinase is activated phosphorylates initiation factor eIF2 eIF2-GTP delivers initiator tRNA to ribosome, then is hydorlyzed to eIF2-GDP phosphorylated eIF2 cannot exchange GDP for GTP and therefore cannot initiate the next round of translation Post-Translational Control o o o o o o For protein stability not well understood Protein stability may be determined by the amino acids on the Nterminus Degradation of proteins is carried out within hollow, cylindrical proteasomes Proteasomes recognize proteins linked to ubiquitin Ubiquitin is transferred by ubiquitin ligases to proteins being degraded Once polyubiqwuitinated, a protein is recognized by the cap of the proteasome and degraded 7...
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