tension.jpgIt 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).