voltage-clamp-fluorometry-xenopus-oocytes-using-fluorescent-unnatural

Voltage-clamp Fluorometry in <em>Xenopus</em> Oocytes Using Fluorescent Unnatural Amino Acids

Voltage-Clamp Fluorometry (VCF) has been the technique of choice to investigate the structure and function of electrogenic membrane proteins where ...

University of Montréal

Atomic-scale models on the gating mechanism of voltage-gated potassium channels (Kv) are based on linear interpolations between static structures of their initial and final state derived from crystallography and molecular dynamics simulations, and, thus, lack dynamic structural information. The lack of information on dynamics and intermediate states makes it difficult to associate the structural with the dynamic functional data obtained with electrophysiology. Although voltage-clamp fluorometry fills this gap, it is limited to sites extracellularly accessible, when the key region for gating is located at the cytosolic side of the channels. Here, we solved this problem by performing voltage-clamp fluorometry with a fluorescent unnatural amino acid. By using an orthogonal tRNA-synthetase pair, the fluorescent unnatural amino acid was incorporated in the Shaker voltage-gated potassium channel at key regions that were previously inaccessible. Thus, we defined which parts act independently and which parts act cooperatively and found pore opening to occur in two sequential transitions.

Dynamics of internal pore opening in K(V) channels probed by a fluorescent unnatural amino acid.

With the rapid development of a continuously growing selection of unnatural amino acids (UAAs), UAA insertion becomes increasingly popular for investigating proteins. However, it can prove problematic to ensure the homogeneity of the expressed proteins, when homogeneity is compromised by "leak expression". Here, we show that leak expression may be mediated by reinitiation and can result in unwanted proteins when stop codons for UAA insertion are mutated into the N-terminus of proteins. We demonstrate that up to 25% of leak expression occurs through reinitiation in the Shaker-Kv channel when stop codons are located within the first 70 amino acids. Several non-canonical start codons were identified as translation reinitaition sites, and by removing the start codons, we were able to decrease leak expression to less than 1%. Our study emphasizes the need to carefully inspect for leak expression when inserting UAAs and demonstrates how leak expression can be eliminated.

Reinitiation at non-canonical start codons leads to leak expression when incorporating unnatural amino acids.

The mutation F184C in Kv1.1 leads to development of episodic ataxia type I (EA1). Although the mutation has been said to alter activation kinetics and to lower expression, we show here that the underlying molecular mechanisms may be more complex. Although F184 is positioned in the "peripheral" S1 helix, it occupies a central position in the 3D fold. We show in cut-open oocyte voltage-clamp recordings of gating and ionic currents of the Shaker Kv channel expressed in Xenopus oocytes that F184 not only interacts directly with the gating charges of the S4, but also creates a functional link to the selectivity filter of the neighboring subunit. This link leads to impaired fast and slow inactivation. The effect on fast inactivation is of an allosteric nature considering that fast inactivation is caused by a linked cytosolic ball peptide. The extensive effects of F184C provide a new mechanism underlying EA.

Article	Year
Dynamics of internal pore opening in K(V) channels probed by a fluorescent unnatural amino acid. Proceedings of the National Academy of Sciences of the United States of America\| PubMed ID: 23630265	2013
Reinitiation at non-canonical start codons leads to leak expression when incorporating unnatural amino acids. Scientific reports\| PubMed ID: 26153354	2015
A Disease Mutation Causing Episodic Ataxia Type I in the S1 Links Directly to the Voltage Sensor and the Selectivity Filter in Kv Channels. The Journal of neuroscience : the official journal of the Society for Neuroscience\| PubMed ID: 26338330	2015

Tanja Kalstrup

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