Linking a lipid to a protein helps to provide an "anchor" for the protein inside of the lipid bilayer of cellular membranes. Examples include the following:
1. Acylation - transfer of single saturated acyl groups. These are primarily myristate (C14) linked as an amide to N-terminal glycine and palmitate (C16) linked as a thioester to cysteine. A large number of viral proteins, membrane proteins, and proteins involved in signal transduction are acylated in this manner.
2. Prenylation - transfer of C15 or C20 groups from intermediates in cholesterol biosynthesis to cysteine residues four positions from the C-terminus of the protein. (Figure 19.10).
3. Esterification to cholesterol - recently the "hedgehog" family of proteins regulating differentiation has been shown to be esterified with cholesterol near the C terminus. Signaling by hedgehog proteins in development plays a significant role in defining the polarity (orientation) of many tissue types in vertebrate and invertebrate organisms.
4. Linkage of a cell surface protein to a glycosylated form of phosphatidylinositol. Figure 19.9 shows a proposed mechanism by which this may occur.