escence domain of JNJ-7777120 site spartin to 
cardiolipin, a mitochondrial phospholipid; this might be a major means by which spartin associates with the mitochondria. Human neuroblastoma cells depleted of spartin and cortical neurons obtained from Spg20 KO mice showed depolarized mitochondrial membrane. In addition, knockdown of spartin April 2011 | Volume 6 | Issue 4 | e19290 Spartin Regulates Mitochondrial Ca2+ Homeostasis reduced mitochondrial Ca2+ influx upon thapsigargin stimulation. Our findings suggest that spartin is an important player in the physiological function of mitochondria and that lack of spartin’s expression in patients with Troyer syndrome might cause impaired mitochondrial calcium handling, which could contribute to the pathophysiology of the disease. The subcellular localization of spartin in mitochondria has been controversial. Two studies by Byrne’s laboratory have shown, by immunofluorescence and immunoblotting, that overexpressed and endogenous spartin is present in the mitochondria. However, a study by Eastman and colleagues did not replicate those findings. By using both immunofluorescence and differential centrifugation of cells expressing endogenous spartin, we found that spartin localized to cytoplasm and mitochondria. Furthermore, using a biochemical assay we showed that the Cterminus of spartin is responsible for its association with mitochondria. These findings agree with an earlier immunofluorescence study demonstrating that the C-terminus of overexpressed spartin colocalizes with mitochondria. Proteins that are not permanently located in the mitochondria due to the lack of a mitochondrial targeting signal associate with the mitochondria by two major, non-mutually exclusive mechanisms: 1) they bind to the proteins that reside in the mitochondria and/or 2) they bind to phospholipids enriched in the mitochondrial membranes. We found that spartin, through its plant-related senescence domain, binds to cardiolipin, a phospholipid that is present mostly in the inner, and to a lesser degree in the outer mitochondrial membrane. Our experiments examining the topology of spartin revealed that it associates with the outer mitochondria membrane. This finding together with the presence of a large pool of spartin in the cytoplasm, suggests that spartin might transiently bind to cardiolipin and/or protein located on the outer mitochondrial membrane. This would also be in agreement with our previous findings showing that spartin is a highly mobile protein. Spartin is a multifunctional protein and localizes to many subcellular compartments. The localization of spartin to some specific organelles is transient and becomes evident after cells are treated with compounds that facilitate a particular physiological change. For example, treatment of cells with epidermal growth factor, which induces synchronized endocytosis in serum-starved cells, facilitates distribution of spartin to the endosomes. When cells are treated with oleic acid, it causes the formation of lipid droplets and causes spartin to relocate from the cytoplasm to those lipid droplets. In cells grown in regular medium with serum, endogenous spartin has been reported to localize to transGolgi, midbodies during cell division, and to mitochondria. Recently, it was reported that spartin affects lipid droplet turnover, and in the present study we show that spartin also regulates calcium uptake to mitochondria. Thus, spartin’s localization to the lipid droplets and mitochondria ha
