Sensitive to change

Voltage-dependent K+ channels open and conduct ions in response to changes in membrane voltage. But, how do these channels 'sense' membrane voltage and how do they open in response to a particular voltage? Work by MacKinnon and colleagues now published in Nature provides new insights. They determined the 3.2-Å-resolution crystal structure of full-length KvAP — a voltage-dependent K+ channel from Aeropyrum pernix — and the 1.9-Å-resolution crystal structure of the isolated voltage-sensor domain, both in complex with monoclonal Fab fragments.

The KvAP channel is a tetramer, and its subunits are composed of six hydrophobic segments (S1–S6). S5 and S6 of each subunit line the pore and determine ion selectivity, whereas S1–S4 form the voltage sensors. Although KvAP has a canonical K+ pore, the authors saw that the surrounding voltage sensors have an unexpected structure. S1 and S2 form a layer of helices outside S5, and S3 and S4 are located on the pore's outer perimeter. Surprisingly, the S3 segment is actually composed of two helices (S3a and S3b), and the S3b–S4 helix–turn–helix structures form so-called 'voltage-sensor paddles'. The paddles are mainly hydrophobic, except for S4 arginine residues, and they can move with respect to the pore. A 'flip' movement of these paddles in response to membrane voltage changes would be linked to movements of the pore through the S4–S5 linker. And, in another study published in the same issue of Nature, MacKinnon and co-workers describe how far these paddles move when the pore opens in response to membrane depolarization. REFERENCE Jiang, Y. et al. X-ray structure of a voltage-dependent K+ channel. Nature 423, 33–41 (2003)

High fidelity

Aminoacyl-tRNA synthetases attach amino acids to their cognate tRNA, and the precision of this reaction is crucial for the fidelity of protein synthesis. To enhance fidelity, some of these synthetases use hydrolysis to 'edit' incorrectly activated amino acids (aminoacyl-adenylates; 'pre-transfer editing') or mischarged tRNAs (aminoacyl-tRNAs; 'post-transfer editing'). And, because these substrates are distinct, it was thought that there must be separate amino-acid binding sites for these editing reactions. However, in Molecular Cell, Martinis, Cusack and colleagues now show that these binding sites essentially overlap.

The authors determined the crystal structure of leucyl-tRNA synthetase bound to analogues of pre- and post-transfer editing substrates. They found that the synthetase's editing active site binds these substrates so that the common groups — the adenine and amino-acid moiety — are bound in the same specificity pockets and are recognized in the same way. A conserved aspartic acid in the synthetase interacts with the α-amino group of the non-cognate amino acid in both substrates, and this aspartic acid is essential for editing. It proofreads amino-acid side chains in the discrimination pocket, and also positions substrates for hydrolysis by water (the authors propose that the editing-site residues have no direct catalytic effect). This work shows “...the economy by which a single active site accommodates two distinct substrates in a proofreading process critical to the fidelity of protein synthesis”. REFERENCE Lincecum, T. L. et al. Structural and mechanistic basis of pre- and posttransfer editing by leucyl-tRNA synthetase. Mol. Cell 11, 951–963 (2003)