A budding yeast-centric view of oxysterol binding protein family function.

The Trans Golgi Network (TGN)/endosomal system is a sorting center for cargo brought via the anterograde secretory pathway and the endocytic pathway that internalizes material from the plasma membrane. As many of the cargo that transit this central trafficking hub are components of key homeostatic signaling pathways, TGN/endosomes define a critical signaling hub for cellular growth control. A particularly interesting yet incompletely understood aspect of regulation of TGN/endosome function is control of this system by two families of lipid exchange/lipid transfer proteins. The phosphatidylinositol transfer proteins promote pro-trafficking phosphoinositide (i.e. phosphatidylinositol-4-phosphate) signaling pathways whereas proteins of the oxysterol binding protein family play reciprocal roles in antagonizing those arms of phosphoinositide signaling. The precise mechanisms for how these lipid binding proteins execute their functions remain to be resolved. Moreover, information regarding the coupling of individual members of the oxysterol binding protein family to specific biological activities is particularly sparse. Herein, we review what is being learned regarding functions of the oxysterol binding protein family in the yeast model system. Focus is primarily directed at a discussion of the Kes1/Osh4 protein for which the most information is available.