Release of cholesterol-rich particles from the macrophage plasma membrane during movement of filopodia and lamellipodia.

Cultured mouse peritoneal macrophages release large numbers of ~30-nm cholesterol-rich particles. Here, we show that those particles represent fragments of the plasma membrane that are pulled away and left behind during the projection and retraction of filopodia and lamellipodia. Consistent with this finding, the particles are enriched in proteins found in focal adhesions, which attach macrophages to the substrate. The release of particles is abolished by blocking cell movement (either by depolymerizing actin with latrunculin A or by inhibiting myosin II with blebbistatin). Confocal microscopy and NanoSIMS imaging studies revealed that the plasma membrane-derived particles are enriched in 'accessible cholesterol' (a mobile pool of cholesterol detectable with the modified cytolysin ALO-D4) but not in sphingolipid-sequestered cholesterol [a pool detectable with ostreolysin A (OlyA)]. The discovery that macrophages release cholesterol-rich particles during cellular locomotion is likely relevant to cholesterol efflux and could contribute to extracellular cholesterol deposition in atherosclerotic plaques

Shared functions of plant and mammalian StAR-related lipid transfer (START) domains in modulating transcription factor activity

Abstract Background Steroidogenic acute regulatory protein (StAR)-related lipid transfer (START) domains were first identified from mammalian proteins that bind lipid/sterol ligands via a hydrophobic pocket. In plants, predicted START domains are predominantly found in homeodomain leucine zipper (HD-Zip) transcription factors that are master regulators of cell-type differentiation in development. Here we utilized studies of Arabidopsis in parallel with heterologous expression of START domains in yeast to investigate the hypothesis that START domains are versatile ligand-binding motifs that can modulate transcription factor activity. Results Our results show that deletion of the START domain from Arabidopsis Glabra2 (GL2), a representative HD-Zip transcription factor involved in differentiation of the epidermis, results in a complete loss-of-function phenotype, although the protein is correctly localized to the nucleus. Despite low sequence similarly, the mammalian START domain from StAR can functionally replace the HD-Zip-derived START domain. Embedding the START domain within a synthetic transcription factor in yeast, we found that several mammalian START domains from StAR, MLN64 and PCTP stimulated transcription factor activity, as did START domains from two Arabidopsis HD-Zip transcription factors. Mutation of ligand-binding residues within StAR START reduced this activity, consistent with the yeast assay monitoring ligand-binding. The D182L missense mutation in StAR START was shown to affect GL2 transcription factor activity in maintenance of the leaf trichome cell fate. Analysis of in vivo protein–metabolite interactions by mass spectrometry provided direct evidence for analogous lipid-binding activity in mammalian and plant START domains in the yeast system. Structural modeling predicted similar sized ligand-binding cavities of a subset of plant START domains in comparison to mammalian counterparts. Conclusions The START domain is required for transcription factor activity in HD-Zip proteins from plants, although it is not strictly necessary for the protein’s nuclear localization. START domains from both mammals and plants are modular in that they can bind lipid ligands to regulate transcription factor function in a yeast system. The data provide evidence for an evolutionarily conserved mechanism by which lipid metabolites can orchestrate transcription. We propose a model in which the START domain is used by both plants and mammals to regulate transcription factor activity

Shared functions of plant and mammalian StAR-related lipid transfer (START) domains in modulating transcription factor activity

Background: Steroidogenic acute regulatory protein (StAR)-related lipid transfer (START) domains were first identified from mammalian proteins that bind lipid/sterol ligands via a hydrophobic pocket. In plants, predicted START domains are predominantly found in homeodomain leucine zipper (HD-Zip) transcription factors that are master regulators of cell-type differentiation in development. Here we utilized studies of Arabidopsis in parallel with heterologous expression of START domains in yeast to investigate the hypothesis that START domains are versatile ligand-binding motifs that can modulate transcription factor activity. Results: Our results show that deletion of the START domain from Arabidopsis Glabra2 (GL2), a representative HD-Zip transcription factor involved in differentiation of the epidermis, results in a complete loss-of-function phenotype, although the protein is correctly localized to the nucleus. Despite low sequence similarly, the mammalian START domain from StAR can functionally replace the HD-Zip-derived START domain. Embedding the START domain within a synthetic transcription factor in yeast, we found that several mammalian START domains from StAR, MLN64 and PCTP stimulated transcription factor activity, as did START domains from two Arabidopsis HD-Zip transcription factors. Mutation of ligand-binding residues within StAR START reduced this activity, consistent with the yeast assay monitoring ligand-binding. The D182L missense mutation in StAR START was shown to affect GL2 transcription factor activity in maintenance of the leaf trichome cell fate. Analysis of in vivo protein–metabolite interactions by mass spectrometry provided direct evidence for analogous lipid-binding activity in mammalian and plant START domains in the yeast system. Structural modeling predicted similar sized ligand-binding cavities of a subset of plant START domains in comparison to mammalian counterparts. Conclusions: The START domain is required for transcription factor activity in HD-Zip proteins from plants, although it is not strictly necessary for the protein’s nuclear localization. START domains from both mammals and plants are modular in that they can bind lipid ligands to regulate transcription factor function in a yeast system. The data provide evidence for an evolutionarily conserved mechanism by which lipid metabolites can orchestrate transcription. We propose a model in which the START domain is used by both plants and mammals to regulate transcription factor activity

Characterizing the stabilizing effect of the putative kinase Coq8 and the function of the Coq9 polypeptide in yeast coenzyme Q biosynthesis

Coenzyme Q (Q) is an essential lipid in cellular energy metabolism, but its biosynthesis is not fully understood. Q functions as an electron carrier in the mitochondrial respiratory chain and as a lipid-soluble antioxidant. Q biosynthesis in yeast Saccharomyces cerevisiae requires a multi-subunit Coq polypeptide complex composed of the Coq3−Coq9 polypeptides, but the function of several Coq polypeptides is unknown, including Coq9. Deletion of any of the COQ3−COQ9 genes leads to the decreased steady state of other Coq polypeptides. The over-expression of the putative kinase, Coq8, in some of the yeast coq null mutants, restored steady state levels of Coq polypeptides to near wild-type levels and led to the production of late-stage Q intermediates. In this dissertation, the following chapters summarize four projects on Q biosynthesis: Chapter 2 investigates whether Coenzyme Q6 supplementation or over-expression of Coq8 stabilizes high molecular mass Coq polypeptide complexes. Based on our findings, we proposed a new model for the complex, which we called the CoQ-synthome. In Chapter 3, the characterization of Coq9 function is described. We conclude that Coq9 is required for the function of Coq6 and Coq7 and for the removal of the nitrogen substituent from Q-intermediates derived from para-aminobenzoic acid. The functional role of human Coq9 in Q10 biosynthesis is not understood. In Chapter 4 we found that human COQ9 rescues the growth of a temperature-sensitive yeast coq9 mutant, TS19, on non-fermentable carbon source and increases the content of Q6, possibly by increasing the Q biosynthesis from 4-hydroxybenzoic acid (4HB). Chapter 5 demonstrates that para-coumarate is a ring precursor for Q biosynthesis in E. coli, S. cerevisiae, and human cells. This work aids our understanding of Q biosynthesis and suggests new approaches that may enhance Q biosynthesis and function in human disease