Calcium is a major regulator and signal in countless cellular processes. Mitochondria participate by taking in and storing calcium, which helps buffer the amount of free calcium in the cell. A group recently became the first to identify one of the proteins important for mitochondrial calcium uptake. Images above show this protein, called MICU1 (green), localizing onto mitochondria (red); the merged image (right) shows yellow where the two signals are colocalized.
Reference: Fabiana Perocchi, Vishal M. Gohil, Hany S. Girgis, X. Robert Bao, Janet E. McCombs, Amy E. Palmer, and Vamsi K. Mootha. Adapted by permission from Macmillan Publishers Ltd: Nature 467: 291-296, copyright 2010. Paper can be found here.