Calibrating in vivo tension sensor
It has become increasingly clear that the tension across proteins can control the cellular fate but until recently it had not been possible to determine the tension applied across a protein in vivo. The group of Martin Schwartz at University of Virginia developed an in vivo tension sensor where a protein motif from a spider silk protein is labeled with two genetically encoded proteins of different colors at the two ends. By inserting this sensor in the middle of a protein called vinculin, they were able to show that a change in tension across the vinculin protein can be detected as a change in fluorescence resonance energy transfer (FRET) between the two fluorescent proteins. The groups of Taekjip Ha and Steve Sligar designed a scheme to calibrate the tension sensor. By using the single molecule fluorescence-force spectroscopy instrument developed in the Ha group (Hohng et al, Science, 318, 279-283, 2007), CPLC students Michael Brenner and Ruobo Zhou built a precise mapping between the FRET efficiency and force. They found that the tension sensor is most sensitive to the force range 1-5 pN. Combined with cellular FRET imaging, the vinculin protein of a migrating cell is under about 2.5 pN of force (Grashoff et al, Nature, 466, 263-266, 2010).