A tether (gray) holds together mitochondria (red) and the ER (yellow).

Mitochondria and ER membranes are directly tethered to one another, Csordás et al. report on page 915. Changes in tether length make the cell more or less vulnerable to apoptotic triggers.

Indirect evidence suggested that the mitochondria and ER were physically tied to one another, but what such a connection might be was obscure. Using electron tomography, Csordás et al. saw thin threads that ran between the organelles, ranging in size from 6 to 15 nm at the smooth ER, and 19 to 30 nm at the rough ER.

Limited protease digestion lengthened the ties and made the mitochondria less sensitive to Ca2+ release from the ER. By contrast, when the group engineered a 5-nm linker to narrow the gap between the organelles, mitochondria took in apoptosis-inducing amounts of Ca2+....

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