Optical force measurements utilizing Lanthanide Binding Tags

Abstract

The Lanthanide Binding Tag (LBT) is a motif comprised of 17 amino-acids with the sequence YIDTNNDGWYEGDELLA. The LBT has been crystallized in the presence of Tb3+ and thus it is known that this motif forms a loop in which the center is occupied by the Tb3+ ion, with the ion held in place by negatively charged amino-acids. Comparison with the highly related structure of an EF-hand revealed that only in the case of the LBT, water molecules are completely excluded from the interior of the loop (Nitz et al, 2004). This explains the high quantum yield of Tb3+ bound to the LBT.However, when encoded into a large “host molecule” the environment of the LBT is more constrained and it is possible that if inserted in the correct position, it will sense forces originating from conformational changes within the host. The forces exerted by these conformational changes may lead to a deformation or an unfolding of the motif, which would consequently change the emission properties of the bound Tb3+ ion. Here we show that Tb3+ emission from LBTs inserted on a particular position of the voltage sensor of Shaker K channels can be quenched by voltage dependent conformational changes. In order to understand this phenomenon, and with the attempt to calibrate this system for optical force measurements, we have employed molecular dynamic simulations. In these simulations an artificial force was applied to the ends of the LBT motif and the probability of water molecules interacting with the Tb3+ ion as a function of the applied force is used as a parameter related to the observed quenching of the emission.Supported by AHA 07257632(WS) and by NIH GM30376(FB) and GM68044(AMC).