PSI Structural Biology Knowledgebase

PSI | Structural Biology Knowledgebase
Header Icons
E-Collection

Related Articles
Drug Discovery: Solving the Structure of an Anti-hypertension Drug Target
July 2015
Retrospective: 7,000 Structures Closer to Understanding Biology
July 2015
Families in Gene Neighborhoods
June 2015
Channels and Transporters: BEST in Show
April 2015
Channels and Transporters: Reorienting a Peptide in the Pocket
April 2015
Ryanodine Receptor
April 2015
Protein Folding and Misfolding: It's the Journey, Not the Destination
March 2015
Protein Folding and Misfolding: Refolding in Membrane Mimetic
March 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
Mitochondrion: Flipping for UCP2
December 2014
Transmembrane Spans
December 2014
Glucagon Receptor
April 2014
Membrane Proteome: A Cap on Transport
April 2014
Membrane Proteome: Microcrystals Yield Big Data
April 2014
Membrane Proteome: Pumping Out Heavy Metal
April 2014
Design and Discovery: Virtual Drug Screening
January 2014
G Proteins and Cancer
November 2013
Drug Discovery: Antidepressant Potential of 6-NQ SERT Inhibitors
October 2013
Drug Discovery: Modeling NET Interactions
October 2013
Microbiome: Solid-State NMR, Crystallized
September 2013
CAAX Endoproteases
August 2013
Membrane Proteome: A Funnel-like Viroporin
August 2013
Membrane Proteome: GPCR Substrate Recognition and Functional Selectivity
August 2013
Membrane Proteome: Making DNA Nanotubes for NMR Structure Determination
August 2013
Membrane Proteome: Unveiling the Human α-helical Membrane Proteome
August 2013
Cell-Cell Interaction: Magic Structure from Microcrystals
March 2013
Cell-Cell Interaction: Nanoparticles in Cell Camouflage
March 2013
Membrane Proteome: Capturing Multiple Conformations
December 2012
Membrane Proteome: Soft Sampling
December 2012
Membrane Proteome: Sphingolipid Synthesis Selectivity
December 2012
Membrane Proteome: Tuning Membrane Protein Expression
December 2012
Cytochrome Oxidase
November 2012
Membrane Proteome: Building a Carrier
November 2012
Membrane Proteome: Every Protein Has Its Tag
November 2012
Membrane Proteome: Specific vs. Non-specific weak interactions
November 2012
Membrane Proteome: The ABCs of Transport
November 2012
Bacterial Phosphotransferase System
October 2012
Insert Here
October 2012
Solute Channels
September 2012
To structure, faster
August 2012
Pocket changes
July 2012
Predictive protein origami
July 2012
G Protein-Coupled Receptors
May 2012
Twist to open
March 2012
Anchoring's the way
February 2012
Overexpressed problems
February 2012
Gentle membrane protein extraction
January 2012
Docking and rolling
October 2011
A fragmented approach to membrane protein structures
September 2011
Raising a glass to GLIC
August 2011
Sugar transport
June 2011
A2A Adenosine Receptor
May 2011
TrkH Potassium Ion Transporter
April 2011
Subtly different
March 2011
A new amphiphile for crystallizing membrane proteins
January 2011
CXCR4
January 2011
Guard cells pick up the SLAC
December 2010
ABA receptor diversity
November 2010
COX inhibition: Naproxen by proxy
November 2010
Zinc Transporter ZntB
July 2010
Formate transporter or channel?
March 2010
Tips for crystallizing membrane proteins in lipidic mesophases
February 2010
Urea transporter
February 2010
Five good reasons to use single protein production for membrane proteins
January 2010
Membrane proteins spotted in their native habitat
January 2010
Spot the pore
January 2010
Get3 into the groove
October 2009
GPCR subunits: Separate but not equal
September 2009
GPCR modeling: any good?
August 2009
Surviving in an acid environment
August 2009
Tips for crystallizing membrane proteins
June 2009
You look familiar: the Type VI secretion system
June 2009
Bacterial Leucine Transporter, LeuT
May 2009
Aquaglyceroporin
March 2009
Death clusters
March 2009
Protein nanopores
March 2009
Transporter mechanism in sight
February 2009
A pocket guide to GPCRs
December 2008
Tuning membrane protein overexpression
October 2008
Blocking AmtB
September 2008

Research Themes Membrane proteins

Channels and Transporters: BEST in Show

SBKB [doi:10.1038/sbkb.2015.6]
Featured Article - April 2015
Short description: Crystal structures of bestrophin ion channels unravel the molecular basis of retinopathies.

Top-down (left) and side (right) views of the bacterial channel KpBest (PDB 4WD8). Figure from ref. 1, reprinted with permission from AAAS.

Bestrophins are calcium-activated chloride channels that are broadly distributed in metazoan tissues and especially abundant in the retinal pigment epithelium. Four human bestrophins (BEST1–4) are known and sequence-related prokaryotic proteins have been identified. While the physiological roles of bestrophins have yet to be fully determined, mutations of human BEST1 are linked to retinopathies such as vitelliform macular dystrophy, or Best disease. To shed light on bestrophin structure and function, Hendrickson and colleagues (PSI NYCOMPS) have solved the crystal structure of a bacterial ortholog of bestrophin from Klebsiella pneumoniae (KpBest, PDB 4WD8) at 2.3-Å resolution, and Long and colleagues have determined the structure of chicken BEST1 (PDB 4RDQ) at 2.85-Å resolution.

Both the eukaryotic and prokaryotic channels are homopentameric (thus clarifying bestrophin stoichiometry) and share a unique architecture consisting of a transmembrane region to which each protomer contributes four predominantly alpha-helical segments, and a large cytoplasmic region, also mostly alpha helical. The ion-conducting pore spans the entire length of BEST1: it begins at the extracellular side with a funnel-shaped vestibule that narrows midway through the transmembrane region to form a ∼6 Å-wide neck spanning three turns of the α2 helix, lined by hydrophobic residues (isoleucines and phenylalanines). Following the neck, the pore opens to form a wide inner cavity that spans the cytoplasmic region.

Long and colleagues reconstituted calcium-activated chloride channel function using purified BEST1 protein. From their structural work, they identified five symmetrical Ca2+-binding sites, each formed by acidic residues clustered at the interface between the transmembrane and cytoplasmic regions. The proximity of the Ca2+-binding sites to the neck, which likely constitutes the channel gate, suggests a possible mechanism whereby Ca2+ binding induces a structural rearrangement of the neck aperture, resulting in channel opening.

Hendrickson and colleagues found that, in contrast to eukaryotic bestrophins, KpBest conducts monovalent cations and does not require Ca2+ for activation. Using mutagenesis and electrophysiology analyses, they showed that the pore-lining hydrophobic residues in the neck affect ion selectivity. In particular, mutating Ile66 to the corresponding phenylalanine conserved in eukaryotic BEST1 channels makes KpBest permeable to anions. Long and colleagues identified anion-binding sites, two in the extracellular vestibule and one in the cytoplasmic cavity of each protomer, which may contribute to anion selectivity in eukaryotic bestrophins.

Finally, the mutations in human BEST1 associated with retinopathies are found to occur especially in the Ca2+-binding sites and the pore neck. Hence, these studies suggest that the mutations cause eye disease by altering channel gating and anion permeation.

Cosma Dellisanti

References

  1. T. Yang et al. Structure and selectivity in bestrophin ion channels.
    Science. 346, 355-9 (2014). doi:10.1126/science.1259723

  2. V. K. Dickson, P. Pedi & S. B. Long Structure and insights into the function of a Ca2+-activated Cl channel.
    Nature. 516, 213-8 (2014). doi:10.1038/nature13913

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