Functional validation of Type 2 IRES-ribosome structural contacts via mutagenesis and translation assays

READY TO TEST Score 0.527 $8,500 – $14,500

Problem mapping

Problem: Lack of complementary biochemical validation for the structural findings of viral Type 2 IRES recruitment to ribosomal preinitiation complex

Sub-question: Do the structural observations correlate with functional translation activity in cellular or in vitro systems?

Objective

To validate the functional importance of specific IRES-ribosome structural contacts identified by cryo-EM/X-ray through site-directed mutagenesis and quantitative translation assays.

Readouts

Primary: Relative translation efficiency (luciferase activity normalized to mRNA levels) for wild-type vs. mutant IRES constructs in rabbit reticulocyte lysate (RRL) in vitro translation system
Secondary: Dual-luciferase reporter ratios (IRES-driven Renilla/cap-dependent Firefly) in transfected HEK293T cells for cellular validation
Secondary: RNA binding affinity (KD) of wild-type and mutant IRES to purified 40S ribosomal subunits measured by microscale thermophoresis (MST)

Experimental design

Overview: Generate 8-12 IRES mutants targeting key structural contact residues identified in cryo-EM/crystal structures. Test translation efficiency in parallel using in vitro RRL system and cellular dual-luciferase assays. Correlate functional defects with direct 40S binding affinity measurements.

WP1: Design and generate IRES mutant library (single, double, and compensatory mutations) targeting ribosome contact sites; clone into dual-luciferase bicistronic reporters and in vitro transcription vectors (Weeks 1-3)
WP2: In vitro transcription of capped, polyadenylated mRNAs; quality control by denaturing gel electrophoresis and capillary electrophoresis; RRL translation assays with luciferase readout and RT-qPCR normalization (Weeks 4-7)
WP3: Transfection of dual-luciferase constructs into HEK293T cells; measure Renilla/Firefly ratios at 24h and 48h; Northern blot or RT-qPCR to confirm mRNA stability (Weeks 6-9)
WP4: Purify 40S ribosomal subunits from HEK293T cells; in vitro transcribe fluorescently-labeled IRES RNAs; perform MST binding assays; statistical analysis and correlation with structural contacts (Weeks 8-12)

Controls:

Positive control: Wild-type EMCV or CrPV IRES (established Type 2 IRES) with known high translation efficiency
Negative control: Bicistronic construct with no IRES (empty intercistronic spacer) to measure background cap-independent translation
Technical control: Monocistronic cap-dependent luciferase construct to normalize transfection efficiency and cell viability; mock-transfected cells for background subtraction

Sample size plan: 8-12 IRES mutants plus wild-type and controls; each construct tested in n=4 biological replicates for RRL assays, n=6 biological replicates for cell-based assays (3 independent transfections × 2 time points), and n=3 technical replicates for MST binding measurements

Success criteria:

At least 60% of mutations targeting direct contact residues show >2-fold reduction in translation efficiency (p<0.05 by one-way ANOVA with Dunnett's post-hoc test) compared to wild-type IRES
Pearson correlation coefficient r > 0.6 (p<0.05) between translation efficiency defects and changes in 40S binding affinity (KD) for mutant IRES constructs
Cellular and in vitro translation assays show concordant results (same direction of effect) for >80% of mutants tested

Estimated timeline: 12 weeks

Materials

Item	Supplier	Catalog / ID	Link	Purpose
Rabbit Reticulocyte Lysate System, Nuclease-Treated	Promega	L4960		In vitro translation system for testing IRES-driven translation efficiency
psiCHECK-2 Vector (dual-luciferase bicistronic reporter)	Promega	C8021		Backbone for cloning IRES variants between Renilla and Firefly luciferase coding sequences
Dual-Luciferase Reporter Assay System	Promega	E1960	source	Quantify IRES-driven Renilla and cap-dependent Firefly luciferase activities in cell lysates
HEK293T cells	ATCC	CRL-3216	source	Mammalian cell line for cellular translation reporter assays and 40S ribosome purification
mMESSAGE mMACHINE T7 Transcription Kit	Thermo Fisher	AM1344	source	Generate capped, polyadenylated mRNA transcripts for in vitro translation assays
Q5 Site-Directed Mutagenesis Kit	New England Biolabs	E0554S	source	Introduce point mutations into IRES sequences at structurally-defined contact sites
Luciferase Assay Reagent (LAR)	Promega	E1483	source	Measure luciferase activity from in vitro translation reactions
Monolith NT.115 Microscale Thermophoresis instrument	NanoTemper Technologies	NT.115		Measure binding affinity (KD) between fluorescently-labeled IRES RNA and purified 40S subunits
Monolith His-Tag Labeling Kit RED-tris-NTA 2nd Generation	NanoTemper Technologies	MO-L018		Fluorescent labeling of RNA or ribosomal proteins for MST measurements
Label IT Nucleic Acid Labeling Kit, Cy5	Mirus Bio	MIR3225		Fluorescently label in vitro transcribed IRES RNA for MST binding assays
Sucrose Cushion Ribosome Purification Buffer Set	Custom preparation	N/A		Buffers for ultracentrifugation-based purification of 40S ribosomal subunits (20 mM Tris-HCl pH 7.5, 100 mM KCl, 5 mM MgCl2, 1M sucrose cushion)
SuperScript IV Reverse Transcriptase	Thermo Fisher	18090010	source	RT-qPCR to quantify mRNA levels for normalization of translation efficiency
PowerUp SYBR Green Master Mix	Applied Biosystems	A25742	source	qPCR quantification of luciferase mRNA levels
Lipofectamine 3000 Transfection Reagent	Thermo Fisher	L3000015	source	Transfect dual-luciferase reporter constructs into HEK293T cells
Beckman Coulter Optima XPN-100 Ultracentrifuge with SW41 rotor	Beckman Coulter	A94471	source	Purification of ribosomal subunits through sucrose cushion ultracentrifugation
Agilent 2100 Bioanalyzer with RNA 6000 Nano Kit	Agilent	5067-1511		Quality control of in vitro transcribed mRNA integrity and purity

Direct cost estimate: $8,500 – $14,500 (Includes molecular cloning reagents, in vitro translation kits, cell culture reagents, transfection reagents, luciferase assay reagents, RT-qPCR supplies, RNA labeling kits, and MST consumables. Excludes major equipment (ultracentrifuge, MST instrument, plate readers, qPCR machine), labor costs, and ribosome purification if commercial 40S subunits are purchased (~$3000 additional))

References

Zuker, M. & Stiegler, P. (1981) 'Optimal computer folding of large RNA sequences using thermodynamics and auxiliary information', Nucleic Acids Research, 9(1), pp. 133-148. — Computational prediction of secondary structure changes introduced by mutations to ensure disruption of tertiary contacts without destabilizing overall IRES fold
Pestova, T.V. & Hellen, C.U. (2003) 'Translation elongation after assembly of ribosomes on the Cricket paralysis virus internal ribosomal entry site without initiation factors or initiator tRNA', Genes & Development, 17(2), pp. 181-186. — Established protocol for in vitro translation assays using Type 2 IRES constructs in rabbit reticulocyte lysate
Deniz, N., et al. (2009) 'Translation initiation factors and their relevance in health and disease', Biological Chemistry, 390(5-6), pp. 331-344. — Guidelines for normalization strategies in translation assays and interpretation of IRES-mediated translation efficiency
Khawaja, A., et al. (2015) 'Measuring thermal stability of proteins in cell lysates by microscale thermophoresis', Methods in Molecular Biology, 1261, pp. 253-261. — Optimized protocols for MST-based measurement of RNA-protein binding affinity including buffer optimization and concentration ranges
Easton, L.E., et al. (2009) 'A novel method to engineer bicistronic constructs for the expression of influenza A proteins in cell culture and Xenopus laevis embryos', Nucleic Acids Research, 37(10), e68. — Design principles for bicistronic dual-luciferase reporters to minimize cryptic promoter activity and ensure accurate IRES-dependent translation measurement
Anger, A.M., et al. (2013) 'Structures of the human and Drosophila 80S ribosome', Nature, 497(7447), pp. 80-85. — Protocol for purification of mammalian ribosomal subunits from cultured cells using sucrose gradient ultracentrifugation

Lab handoff checklist

3D structural coordinates (PDB or CIF file) of the IRES-ribosome complex with annotated contact residues and distances <5Å for mutagenesis targeting
Complete nucleotide sequence of the wild-type IRES element with 5' and 3' flanking sequences (50-100 nt) for cloning into expression vectors
List of 8-12 prioritized residue positions for mutagenesis ranked by structural contact surface area and phylogenetic conservation, including proposed substitutions (e.g., charge reversals, alanine scanning)
Primer design file with sequences for site-directed mutagenesis of all target positions including compensatory double mutants
Reference translation efficiency data (if available) for the wild-type IRES from previous studies for benchmark comparison
Cell culture protocols and media requirements specific to the lab (preferred HEK293T culture conditions, passage numbers, transfection optimization data)

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