Fatty acylated proteins as components of intracellular signaling pathways

Influence of 26-hydroxycholesterol on the composition and function of gel-filtered platelets

The influence of 26-hydroxycholesterol (26-OH-CHOL) on the structure and function of gel-filtered rat platelets, as a model membrane, was studied in vitro. Its influence on structure was determined by a fatty acid and a phospholipid analysis of the platelet lipids and on function by the cytoplasmic calcium concentration of the platelets exposed to increasing concentrations of 26-OH-CHOL for various periods of time. The intracellular free calcium (Ca2+)i of the gel-filtered rat platelets was monitored by a fluorescent probe (quin 2) after incubation in a 37 degrees C water bath with 1 mM Ca2+ and 20 microM quin 2/AM. The presence of 26-OH-CHOL in the incubation media changed both the phospholipid composition and the mixed fatty acid composition in the membrane and increased the intracellular free Ca2+ level of the platelets. As the incubation of platelets with cholesterol (CHOL) or esterified 26-OH-CHOL did not increase intracellular Ca2+ levels, these results indicate that the hydrophilic free 26-hydroxy group in 26-OH-CHOL may have influenced the enzymes catalyzing the synthesis of the phospholipid in the platelet membrane so as to allow it to become more and more "leaky" to Ca2+. Such a fundamental change in membrane structure and function may be responsible for the development of atherosclerosis in the intimal layer of the coronary arteries.

On the principles of functional ordering in biological membranes

Integrating the available data on lipid-protein interactions and ordering in lipid mixtures allows to emanate a refined model for the dynamic organization of biomembranes. An important difference to the fluid mosaic model is that a high degree of spatiotemporal order should prevail also in liquid crystalline, "fluid" membranes and membrane domains. The interactions responsible for ordering the membrane lipids and proteins are hydrophobicity, coulombic forces, van der Waals dispersion, hydrogen bonding, hydration forces and steric elastic strain. Specific lipid-lipid and lipid-protein interactions result in a precisely controlled yet highly dynamic architecture of the membrane components, as well as in its selective modulation by the cell and its environment. Different modes of organization of the compositionally and functionally differentiated domains would correspond to different functional states of the membrane. Major regulators of membrane architecture are proposed to be membrane potential controlled by ion channels, intracellular Ca2+, pH, changes in lipid composition due to the action of phospholipase, cell-cell coupling, as well as coupling of the membrane with the cytoskeleton and the extracellular matrix. Membrane architecture is additionally modulated due to the membrane association of ions, lipo- and amphiphilic hormones, metabolites, drugs, lipid-binding peptide hormones and amphitropic proteins. Intermolecular associations in the membrane and in the membrane-cytoskeleton interface are further selectively controlled by specific phosphorylation and dephosphorylation cascades involving both proteins and lipids, and regulated by the extracellular matrix and the binding of growth factors and hormones to their specific receptor tyrosine kinases. A class of proteins coined architectins is proposed, as a notable example the pp60src kinase. The functional role of architectins would be in causing specific changes in the cytoskeleton-membrane interface, leading to specific configurational changes both in the membrane and cytoskeleton architecture and corresponding to (a) distinct metabolic/differentiation states of the cell, and (b) the formation and maintenance of proper three dimensional membrane structures such as neurites and pseudopods.

Purification of a synthetic myristylated peptide by counter-current chromatography

A preparative purification of myristyl-Gly-Asn-Ile-Phe-Ala-Asn-Leu-Phe-Lys-Gly-Leu-Phe-Gly-Lys-Lys-Glu -NH2 was accomplished using the multi-coil counter-current chromatograph. A partition coefficient was determined in the n-butanol-acetic acid-water (4:1:5) system. Chromatographic runs were made in this system and one modified with ethyl acetate. The peptide material showed anomalous elution behavior due to its surfactant properties. It was found that by loading the sample exclusively in the stationary phase, satisfactory retention of the compound occurred. Finally, conditions utilizing the upper phase as the mobile phase successfully separated the impurities.

Use of a cell-free system to identify the vaccinia virus L1R gene product as the major late myristylated virion protein M25

A 25-kDa vaccinia virus (VV) virion protein, designated M25, is modified in vivo by covalent addition of myristic acid. The predicted amino acid sequences of all VV open reading frames which have been reported were searched for the sequence M-G-X-X-X-(S/T/A), which has been proposed to be the consensus recognition signal for cotranslational modification of proteins by N-myristyltransferase. This conserved signal was found at the amino terminus of a single locus, which corresponded to the leftmost rightward-reading open reading frame (L1R) initiating within the VV HindIII L DNA fragment. By using synthetic oligonucleotides in concert with polymerase chain reaction techniques, a chimeric gene consisting of open reading fram L1R fused to a bacteriophage T7 promoter was constructed and cloned into a plasmid vector. Transcripts derived from the wild-type expression plasmid (designated pL1G1) were translated in vitro in a wheat germ extract to yield a polypeptide with an apparent molecular mass of 25 kDa. This polypeptide was labeled with either [35S]methionine or [3H]myristic acid and comigrated with in vivo-labeled protein M25 on sodium dodecyl sulfate-polyacrylamide gels. Polyclonal antiserum generated in rabbits against a trpE:L1R fusion protein immunoprecipitated a 25-kDa protein labeled either in vitro (the L1R gene product, designated protein L1) or in vivo (from purified VV, protein M25), identifying the M25 protein as the gene product of open reading frame L1R. Chromatographic analysis of the protein L1-bound fatty acid moieties liberated after acid methanolysis resulted in recovery of greater than 99% of the fatty acid as myristate-associated label. Cell-free translation of proteins derived from a set of deletions from the carboxy terminus of the open reading frame L1R suggested that the site of myristylation maps near the amino terminus of protein L1. This hypothesis was supported by cell-free translation of mutant L1R transcripts in which the penultimate glycine codon had been altered by site-directed mutagenesis to encode either an aspartic acid (pL1D1) or alanine (pL1A1) residue. In both cases, the mutant transcripts were translated into a 25-kDa protein which could be labeled in vitro with [35S]methionine but not with [3H]myristic acid. These data demonstrate that VV open reading frame L1R encodes a myristylated protein and provide evidence that the site of modification of protein L1 is the amino-terminal glycine residue.

Myristoylation of heparan sulfate proteoglycan and proteins occurs post-translationally in human colon carcinoma cells

We have recently shown that the heparan sulfate proteoglycan of human colon carcinoma cells is acylated with both myristate and palmitate, two long-chain saturated fatty acids. In this study we show that cycloheximide did not significantly inhibit the incorporation of myristic acid into either proteoglycan or total protein pool. This lack of inhibition occurred under a condition in which protein synthesis was inhibited greater than 90%. Cycloheximide, on the other hand, did not affect the incorporation of [3H]myristic acid into fatty acid nor the intracellular interconversion of myristate to palmitate. Characterization of fatty acyl moiety in the proteoglycan and protein by reverse-phase HPLC revealed that approximately 60% of the covalently bound fatty acids was myristate and the remaining 40% was palmitate. These results indicate that in human colon carcinoma cells myristoylation of heparan sulfate proteoglycan and proteins occurs post-translationally, presumably in the Golgi complex.

Retinoic acid inhibits the myristoylation of a membrane protein in HL-60 cells

Incubation of nondifferentiated HL-60 cells with high specific activity myristic acid results in myristoylation of a 25 KD membrane protein. Myristoylation was inhibited by retinoic acid but not by DMSO, thus indicating that the nonmyristoylated state induced by retinoic acid is causal to rather than the result of the overall differentiation sequel induced. Similarly, incubation of HL-60 cells with high specific activity retinoic acid results in specific retinoylation of a membrane protein of similar molecular mass. The two acylated proteins could however be separated by 2-D gel electrophoresis, thus indicating that inhibition of myristoylation by retinoic acid could not be accounted for by competition between retinoic and myristic acids for the same acylated site.

The gamma subunit of transducin is farnesylated

Protein prenylation with farnesyl or geranylgeranyl moieties is an important posttranslational modification that affects the activity of such diverse proteins as the nuclear lamins, the yeast mating factor mata, and the ras oncogene products. In this article, we show that whole retinal cultures incorporate radioactive mevalonic acid into proteins of 23-26 kDa and one of 8 kDa. The former proteins are probably the "small" guanine nucleotide-binding regulatory proteins (G proteins) and the 8-kDa protein is the gamma subunit of the well-studied retinal heterotrimeric G protein (transducin). After deprenylating purified transducin and its subunits with Raney nickel or methyl iodide/base, the adducted prenyl group can be identified as an all-trans-farnesyl moiety covalently linked to a cysteine residue. Thus far, prenylation reactions have been found to occur at cysteine in a carboxyl-terminal consensus CAAX sequence, where C is the cysteine, A is an aliphatic amino acid, and X is undefined. Both the alpha and gamma subunits of transducin have this consensus sequence, but only the gamma subunit is prenylated. Therefore, the CAAX motif is not necessary and sufficient to direct prenylation. Finally, since transducin is the best understood G protein, both structurally and mechanistically, the discovery that it is farnesylated should allow for a quantitative understanding of this post-translational modification.