(A) Like the original, the optimized tension sensor module contains a spring element (wavy line) attached to two fluorescent proteins (colored cylinders) via linker regions (white circles). However, while the original used cyan and yellow fluorescent proteins, the new version uses a green-red pair. Specifically, a green fluorescent protein called Clover acts as the ‘donor’ (green cylinder), and a red fluorescent protein called mRuby2 acts as the ‘acceptor’ (red cylinder). Excitation of the donor with cyan light causes it to give off a bright green light. If the donor is close enough to the acceptor – for example, because the spring is at rest – the fluorescence from the donor can be transferred to the acceptor via a process called FRET (see main text): the acceptor then emits red light. Dashed arrows of different colors indicate light of different wavelengths. (B) Via genetic engineering, this sensor module can be inserted within proteins of interest. If that protein is put under tension (gray solid arrows), the donor and acceptor proteins are pulled apart. This causes the amount of energy transferred by FRET to decrease, increasing emission of green light from the donor and reducing emission of red light from the acceptor. As such the ratio of emission at these two wavelengths provides a measure of how much the spring is extended, which gives an indication of the forces experienced within the protein of interest.