Spiralin is the most abundant protein of several species of spiroplasmas, helical, motile bacteria pathogenic for arthropods and plants. This amphiphilic protein is anchored to the outer face of the plasma membrane by a lipoylated N-terminal cysteine. Although spiroplasma pathogenicity in mammals is controversial, it was shown that spiralin is highly immunogenic and endowed with immunomodulatory activity. In this paper, we describe a high performance method for the purification of Spiroplasma melliferum spiralin under non-denaturing conditions. The protein was selectively extracted with 3-[(3-cholamidopropyl) dimethylammonio]-1-propyl sulfonate (CHAPS) from the membrane pre-treated with sodium dodecyl-N-sarcosinate (Sarkosyl), and purified to homogeneity by cation-exchange HPLC with an overall yield of ∼60%. Detergent-depleted, water-soluble micelles of spiralin displaying a mean diameter of 170Å, as evidenced by transmission electron microscopy, were obtained by dialysis detergent removal. Circular dichroism spectroscopy and cross immunoprecipitation assay of the purified spiralin strongly suggested that this purification method could retain the structural characteristics of the native spiralin 47f9hwr. The strategy developed to purify spiralin (two successive selective extractions of membrane proteins with mild detergents followed by ion-exchange chromatography) should prove useful for the purification of membrane lipoproteins of other bacteria of the class Mollicutes including different pathogens for humans, animals and plants.
SPX domains control phosphate homeostasis in eukaryotes. Ten genes in yeast encode SPX-containing proteins, among which YDR089W is the only one of unknown function. Here, we show that YDR089W encodes a novel subunit of the Vacuole Transporter Chaperone (VTC) complex that produces inorganic polyphosphate (polyP). PolyP synthesis transfers inorganic phosphate (Pi) from the cytosol into the acidocalcisome- and lysosome-related vacuoles of yeast, from where it can be released again. It was hence proposed to buffer changes in cytosolic Pi concentration (1). Vtc5 physically interacts with the VTC complex and accelerates the accumulation of polyP synthesized by it go to this site. Deletion of VTC5 reduces polyP accumulation in vivo and in vitro. Its overexpression hyper-activates polyP production and triggers the phosphate starvation response via the PHO pathway. Since this Vtc5-induced starvation response can be reverted by shutting down polyP synthesis genetically or pharmacologically, we propose that polyP synthesis rather than Vtc5 itself is a regulator of the PHO pathway. Our observations suggest that polyP synthesis not only serves to establish a buffer for transient drops in cytosolic Pi levels, but that it can actively decrease or increase the steady state of cytosolic Pi.
Cells control the size of their compartments relative to cell volume, but there is also size control within each organelle. Yeast vacuoles neither burst nor do they collapse into a ruffled morphology, indicating that the volume of the organellar envelope is adjusted to the amount of content. It is poorly understood how this adjustment is achieved. We show that the accumulating content of yeast vacuoles activates fusion of other vacuoles, thus increasing the volume-to-surface ratio. Synthesis of the dominant compound stored inside vacuoles, polyphosphate, stimulates binding of the chaperone Sec18/NSF to vacuolar SNAREs, which activates them and triggers fusion. SNAREs can only be activated by lumenal, not cytosolic, polyphosphate (polyP). Control of lumenal polyP over SNARE activation in the cytosol requires the cytosolic cyclin-dependent kinase Pho80-Pho85 and the R-SNARE Nyv1. These results suggest that cells can adapt the volume of vacuoles to their content through feedback from the vacuole lumen to the SNAREs on the cytosolic surface of the organelle.
How proteins migrate through the interconnected organelles of the endolysosomal system is poorly understood. A piece of the puzzle has been added with the identification of a complex of tethering factors that functions in the recycling of proteins towards the cell surface.