Ltc1 gets added to the ER’s contacts

Murley et al. identify a yeast protein that might coordinate the function of vacuoles and mito-chondria by transferring sterols at the sites where these organ-elles are contacted by the ER. The ERMES complex connects the yeast ER to mito-chondria, promoting the exchange of lipids between these organelles and marking the sites of mitochondrial division. Murley et al. looked for proteins that interact with the ERMES complex and identifi ed a previously uncharacterized ER protein called Ylr072w. The protein localized to ER–vacuole as well as ER–mitochondria contacts and was recruited to these sites by Vac8 and Tom70/71, respectively. Because Ylr072w could transfer sterols between artifi cial membranes in vitro, the researchers renamed it Lipid transfer at contact site 1, or Ltc1. The researchers also identifi ed a family of related proteins, conserved from yeast to humans. Certain environmental stresses induce the formation of ergosterol-rich domains in vacuolar membranes. Cells required Ltc1 to form these domains and, when Murley et al. increased the proportion of Ltc1 at ER–vacuole contacts, cells formed ergosterol-rich vacuole domains even in the absence of stress. The function of these membrane domains is unknown, but, because they contain components of the TORC1 signaling complex, they may control the activity of this key metabolic regulator. Whether Ltc1 senses or alters the lipid content of mito-chondria remains unclear, but the protein was required for cell viability in the absence of the ERMES complex protein Mdm34, indicating that Ltc1 separately regulates mitochondrial function. Indeed, by contacting both mitochondria and vacuoles, Ltc1 might coordinate the metabolic activities of the two organelles. Luo et al. describe how the Drosophila ovarian niche limits its own ability to maintain germline stem cells (GSCs) in an undifferentiated state. Stem cells reside in specialized micro-environments, or niches, that prevent them from differentiating prematurely. In the germaria of Drosophila ovaries, GSCs reside next to cap cells that inhibit differentiation by secreting the TGF␤-like molecule Dpp. In other tissues, Dpp can infl uence cell fate over long distances, but, in the ovary, its range is spatially restricted such that, when a GSC divides, one of its daughters is positioned too far away from the cap cells to receive the Dpp signal, and therefore starts to differentiate into an egg. Luo et al. were interested in how escort cells—another component of the ovarian niche—help to limit Dpp's sphere of infl uence. The researchers found that escort cells express the Dpp …