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+.

When wild-type cells were exposed to ER stress, the interorganelle tethers appeared to shorten before the cells entered apoptosis. The researchers speculate that the decreased distance enhances Ca2+ transfer from the ER to the mitochondria by keeping the organelles in immediate proximity. Whether other conditions push the organelles further apart—to safeguard mitochondria and prevent cell death—is unknown as yet.

It is also not yet clear what proteins comprise the tethers, but given their varied lengths, the team predicts that it will not be a single protein. In addition to organelle spacing, tether components might also help to control ER and mitochondria fusion and fission.