PSI Structural Biology Knowledgebase

PSI | Structural Biology Knowledgebase
Header Icons
E-Collection

Related Articles
Community-Nominated Targets
July 2015
Drug Discovery: Solving the Structure of an Anti-hypertension Drug Target
July 2015
Retrospective: 7,000 Structures Closer to Understanding Biology
July 2015
Design and Evolution: Unveiling Translocator Proteins
June 2015
Signaling with DivL
May 2015
Signaling: A Platform for Opposing Functions
May 2015
Signaling: Securing Lipid-Protein Partnership
May 2015
Dynamic DnaK
March 2015
Iron-Sulfur Cluster Biosynthesis
December 2014
Mitochondrion: Flipping for UCP2
December 2014
Mitochondrion: Setting a New TRAP1
December 2014
Power in Numbers
August 2014
Quorum Sensing: A Groovy New Component
August 2014
Quorum Sensing: E. coli Gets Involved
August 2014
iTRAQing the Ubiquitinome
July 2014
Microbiome: The Dynamics of Infection
September 2013
Protein-Nucleic Acid Interaction: A Modified SAM to Modify tRNA
July 2013
Protein-Nucleic Acid Interaction: Versatile Glutamate
July 2013
PDZ Domains
April 2013
Alpha-Catenin Connections
March 2013
Cell-Cell Interaction: A FERM Connection
March 2013
Cell-Cell Interaction: Magic Structure from Microcrystals
March 2013
Cell-Cell Interaction: Modulating Self Recognition Affinity
March 2013
Bacterial Hemophores
January 2013
Archaeal Lipids
December 2012
Membrane Proteome: Capturing Multiple Conformations
December 2012
Lethal Tendencies
October 2012
Symmetry from Asymmetry
October 2012
A signal sensing switch
September 2012
Regulatory insights
September 2012
AlkB Homologs
August 2012
Budding ensemble
August 2012
Targeting Enzyme Function with Structural Genomics
July 2012
The machines behind the spindle assembly checkpoint
June 2012
Chaperone interactions
April 2012
Pilus Assembly Protein TadZ
April 2012
Revealing the Nuclear Pore Complex
March 2012
Topping off the proteasome
March 2012
Twist to open
March 2012
Disordered Proteins
February 2012
Analyzing an allergen
January 2012
Making Lipopolysaccharide
January 2012
Pulling on loose ends
January 2012
Terminal activation
December 2011
The Perils of Protein Secretion
November 2011
Bacterial Armor
October 2011
TLR4 regulation: heads or tails?
October 2011
Ribose production on demand
September 2011
Moving some metal
August 2011
Looking for lipids
July 2011
Ribofuranosyl Binding Protein
June 2011
A molecular switch for neuronal growth
May 2011
Cell wall recycler
May 2011
Added benefits
April 2011
NMR challenges current protein hydration dogma
March 2011
Nitrile Reductase QueF
March 2011
Tip formin
March 2011
Inhibiting factor
February 2011
PASK staying active
February 2011
Tryptophanyl-tRNA Synthetase
February 2011
Regulating nitrogen assimilation
January 2011
Subtle shifts
January 2011
Nitrobindin
December 2010
Function following form
October 2010
tRNA Isopentenyltransferase MiaA
August 2010
Importance of extension for integrin
June 2010
Phytochrome
April 2010
Alg13 Subunit of N-Acetylglucosamine Transferase
February 2010
Hemolysin BL
January 2010
Secretagogin
December 2009
Two-component signaling
December 2009
Network coverage
November 2009
Pseudouridine Synthase TruA
November 2009
Unusual cell division
October 2009
Toxin-antitoxin VapBC-5
September 2009
Salicylic Acid Binding Protein 2
August 2009
Proofreading RNA
July 2009
Ykul structure solves bacterial signaling puzzle
July 2009
Hda and DNA Replication
June 2009
Controlling p53
May 2009
Mitotic checkpoint control
May 2009
Ribonuclease and Ribonuclease Inhibitor
April 2009
The elusive helicase
April 2009
Aquaglyceroporin
March 2009
High-energy storage system
February 2009
A new class of bacterial E3 ubiquitination enzymes
January 2009
Poly(A) RNA recognition
January 2009
Activating BAX
December 2008
Scavenger Decapping Enzyme DcpS
November 2008
Bacteriophage Lambda cII Protein
October 2008
New metal-binding domain
October 2008
Blocking AmtB
September 2008
T-Rex
September 2008
Aspartate Dehydrogenase
August 2008
RNase T
July 2008
Chronophin
May 2008

Research Themes Cell biology

The elusive helicase

PSI-SGKB [doi:10.1038/fa_psisgkb.2009.15]
Featured Article - April 2009
Short description: A combination of NMR, mutagenesis and biochemistry reveals the dynamic nature of the eIF4A/4G/4H helicase complex.Cell 136, 447-470 (2009)

Protein synthesis is tightly controlled, and one of the main steps at which it is regulated is the initiation of mRNA translation. Initiation is usually the rate-limiting step in translation; it is the stage at which the preinitiation complex containing the small (40S) ribosomal subunit assembles and is recruited to the 5′ cap of mRNA. Once recruited, the complex scans along the mRNA in the 3′ direction in search of the start codon.

The four main steps of comparative protein structure modeling: template selection, target–template alignment, model building and model quality evaluation.

Scanning is hampered by secondary structure within the 5′ untranslated region of mRNA, and an ATP-dependent helicase activity is needed to facilitate the binding of the preinitiation complex and improve scanning. The helicase is provided by the eukaryotic initiation factor eIF4A. This has very low helicase activity on its own but is much more efficient when complexed with the accessory proteins eIF4B, eIF4E, eIF4G and eIF4H. eIF4A consists of two helicase domains, both of which bind RNA and ATP.

Despite the general functions of eIF4A, eIF4G and eIF4E having been known for decades, and their individual structures having been solved, the structure of the multiprotein complex has remained elusive. This is probably because contacts within the complex are constantly rearranged as it moves along and unwinds mRNA, which makes crystallization difficult.

Marintchev et al. now reveal the topology of the human eIF4A/4G/4H helicase complex through a combination of NMR, site-directed mutagenesis and biochemical assays. They show that it comprises a dynamic network of multiple weak but specific interactions that are continuously rearranged during the ATP-binding and hydrolysis cycle of the helicase.

They also demonstrate that the stable association of eIF4H with eIF4A and eIF4G requires the presence of ATP. It was already known that the entire complex cycles through three distinct states: ATP-bound, ADP-bound and nucleotide free, but this study indicates that the overall domain orientation remains roughly similar in all three states.

Marintchev et al. probed various interactions between the subunits to show that the accessory proteins modulate the affinity of eIF4A for ATP by interacting with both of its helicase domains and by promoting either the closed, ATP-bound, conformation or the open, nucleotide-free, conformation. They also found that helicase tethers eIF4A to mRNA, and that binding of eIF4H to single-stranded mRNA behind eIF4A prevents mRNA annealing and promotes the unidirectional translocation of eIF4A.

The most striking finding is that the complex contacts the mRNA on both sides of the nucleotides located in the ribosome's decoding sites, and the authors propose that different subunits are located in front of and behind the 40S subunit during scanning. Such a conformation could be achieved by wrapping the mRNA around the 'neck' of the 40S subunit, bringing both ends of the mRNA close together. This would position the complex at the mRNA entry channel in the appropriate location for unwinding RNA secondary structure.

Maria Hodges

References

  1. A. Marintchev et al. Topology and regulation of the human eIF4A/4G/4H helicase complex in translation initiation.
    Cell 136, 447-470 (2009).

Structural Biology Knowledgebase ISSN: 1758-1338
Funded by a grant from the National Institute of General Medical Sciences of the National Institutes of Health