Palmitoylation, lipid anchors

As soon as Ras sticks to the plasma membrane another lipid anchor is attached to it. A putative palmitoyl transferase which is assumed to reside in the... 

Fig. 5. Schematic representation of lipid anchors. The arrow heads and tails represent the N termini and the C termini of mature proteins, respectively, (a) Palmitoylation, (b) Ahnyristoylation, (c) Prenylation, (d) GPI anchor... 

Palmitoylation is, after myristoylation, the most common modification of the a-sub-rmit of the heterotrimeric G-proteins (see chapter 5). The a-subunit of G-proteins can be lipidated in a two-fold marmer, with a myristoic acid and a pahnitoic acid anchor at the N-terminus. It appears in this case that two lipid anchors are necessary to mediate a stable association of the protein with the membrane. The lipidation of cytoplasmic protein tyrosine kinase also includes both myristoylation and palmitoylation. H-Ras protein also requires, apart from C-terminal farnesylation (see below), a pahnitoyl modification in order to bind to the plasma membrane. In all mentioned examples the fatty acid anchors play an essential role in the signal transduction. 

Fig. 9.6. Lipid anchor of the Ras protein. Membrane association of the Ras protein is mediated via a palmitoyl and a farnesyl anchor (see also 3.7).

Most of the biological functions of the members of the Ras superfamily are linked to the cytoplasmic side of the cell membrane, where specific signals are received and transmitted further. Accordingly, Ras superfamily members contain structural features that mediate membrane association and serve as lipid anchors (see Section 3.7). Prenylation, palmitoylation and myristoylation are post-translational modifications frequently found on Ras superfamily members. 

The function of the Ras protein in cellular signal transduction is inseparably bound up with the plasma membrane. The Ras proteins associate with the inner side of the cell membrane with the help of lipid anchors, such as farnesyl residues and palmitoyl residues (see Section 3.7). 

Proteins can be attached to the membrane in a variety of ways. When a protein completely spans the membrane, it is often in the form of an a-helix or jS-sheet. These structures minimize contact of the polar parts of the peptide backbone with the nonpolar lipids in the interior of the bilayer (Figure 8.17). Proteins can also be anchored to the lipids via covalent bonds from cysteines or free amino groups on the protein to one of several lipid anchors. Myristoyl and palmitoyl groups are common anchors (Figure 8.17). 

Lipidation -acylation TV -myristoylation Myristoy-lation S-prenylation Prenylation Palmitoylation Isoprenylation GPI anchors Glypiation... 

Many GPCRs contain one or more conserved cysteine residues within their C-terminal tails, which are modified by covalent attachment of palmitoyl or isoprenyl residues. The palmitoyl moiety is anchored in the lipid bilayer forming a fourth intracellular loop. There is evidence that palmitoylation of a GPCR is a dynamic process and may affect receptor desensitization. 

Fig. 1. Structures of lipids covalently attached to proteins. Panel A shows proteins that are lipidated on cytoplasmi-cally exposed amino acids, whereas panel B shows lipidated proteins in the extracellular leaflet. (A) iV-myristoyl glycine, palmitate thioester-linked to cysteine, farnesyl, or geranylgeranyl (prenyl) thioether-linked to cysteine. (B) A/-palmitoyl cysteine, cholesterol ester-linked to glycine, and a minimal GPI anchor linked to the to amino acid in a GPI-anchored protein. The GPI structure is shown with a diacylglycerol moiety containing two ester-linked fatty acids. Other GPI anchors are based on ceramide, while yet others have monoacylglycerol, a fatty acid ether-linked to glycerol, and/or a fatty acid ester-linked to inositol.

As discussed in Chapter 6, transmembrane proteins usually have a number of posttranslational modifications that provide additional chemical groups to fulfill requirements of the three-dimensional structure. The amino terminus (residues 1-34) extends out of the membrane and has branched high mannose oligosaccharides linked through N -glycosidic bonds to the amide of asparagine (see Fig. 7.10). It is anchored in the lipid plasma membrane by a palmitoyl group that forms a thioester with the SH residue of a cysteine. The COOH terminus, which extends... 

Ras Regulator of gene expression and cell growth, found in mutated oncogenic forms in many human tumors H-Ras, K-Ras, N-Ras, Ral A, Rad, Rap, Rit, Anchored to plasma membrane by farnesyl, palmitoyl, or other lipid groups... 

Lipopeptides, peptides modified with lipid residues which are preferentially bound at the thiol function of cysteine or at the a-amino group of N-terminal amino acids. The a-subunits of heteromeric G proteins and non-tyrosine receptor kinases contain N-myristoylated N-terminal glycine residues together with S-palmitoylation of a neighboring cysteine residue. Lipid moieties are necessary to recruit and anchor peptides and proteins to the membrane. Furthermore, it has been postulated that lipidation of proteins represents an event in signal transduction. The synthesis of lipid-modified peptides is not easy to perform as all coupling and deprotection reactions must be carried out under very mild conditions [S. Moffet et al., EMBO J. 1993, 12, 349 D. Kadereit et al., Chem. Fur./. 2001, 7,1184]. 

The four major types of protein lipidation are A-myristoylation, palmitoylation, prenylation and glycosylphosphatidylinositol-anchor (GPI-anchor) addition (Table 1). 

Anchoring of Polymers. Synthetic polymers with hydrophobic side groups interact with membranes by insertion of these so-called anchors into the bilayer. Polysaccharides such as pullulan and amylopectin, which were derivatized with palmitoyl groups, are sufficiently hydrophobic to insert into lipid bilayers (201). If a sufficient amount of polysaccharide is added after vesicle formation, the... 

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