PSI Structural Biology Knowledgebase

PSI | Structural Biology Knowledgebase
Header Icons
E-Collection

Related Articles
Families in Gene Neighborhoods
June 2015
Signaling: A Platform for Opposing Functions
May 2015
Nuclear Pore Complex: A Flexible Transporter
February 2015
Nuclear Pore Complex: Higher Resolution of Macromolecules
February 2015
Nuclear Pore Complex: Integrative Approach to Probe Nup133
February 2015
Piecing Together the Nuclear Pore Complex
February 2015
iTRAQing the Ubiquitinome
July 2014
CAAX Endoproteases
August 2013
The Immune System: A Strong Competitor
June 2013
The Immune System: Strand Swapping for T-Cell Inhibition
June 2013
PDZ Domains
April 2013
Protein Interaction Networks: Adding Structure to Protein Networks
April 2013
Protein Interaction Networks: Morph to Assemble
April 2013
Protein Interaction Networks: Reading Between the Lines
April 2013
Protein Interaction Networks: When the Sum Is Greater than the Parts
April 2013
Alpha-Catenin Connections
March 2013
Cytochrome Oxidase
November 2012
Bacterial Phosphotransferase System
October 2012
Solute Channels
September 2012
Budding ensemble
August 2012
The machines behind the spindle assembly checkpoint
June 2012
G Protein-Coupled Receptors
May 2012
Revealing the Nuclear Pore Complex
March 2012
Topping off the proteasome
March 2012
Anchoring's the way
February 2012
Reading out regioselectivity
December 2011
An effective and cooperative dimer
November 2011
PDZ domains: sometimes it takes two
November 2011
Raising a glass to GLIC
August 2011
A2A Adenosine Receptor
May 2011
A growing family
February 2011
FERM-ly bound
February 2011
CXCR4
January 2011
Guard cells pick up the SLAC
December 2010
Zinc Transporter ZntB
July 2010
Zinc Transporter ZntB
July 2010
Importance of extension for integrin
June 2010
Spot protein-protein interactions… fast
March 2010
Alg13 Subunit of N-Acetylglucosamine Transferase
February 2010
Urea transporter
February 2010
Two-component signaling
December 2009
ABA receptor...this time for real?
November 2009
Network coverage
November 2009
Get3 into the groove
October 2009
Guanine Nucleotide Exchange Factor Vav1 and Rho GTPase Rac1
October 2009
GPCR subunits: Separate but not equal
September 2009
Proofreading RNA
July 2009
Ribonuclease and Ribonuclease Inhibitor
April 2009
The elusive helicase
April 2009
Click for cancer-protein interactions
December 2008

Research Themes Protein-protein interactions

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