A proteolipid is a proteincovalently linked to lipid molecules, which can be fatty acids or sterols. The process of such a linkage is known as protein lipidation, and falls into the wider category of acylation and post-translational modification. Proteolipids are abundant in brain tissue, and are also present in many other animal and plant tissues. They are proteins covalenently bound to fatty acid chains, often granting them an interface for interacting with biological membranes. They are not to be confused with lipoproteins, a kind of sphericalassembly made up of many molecules of lipids and some apolipoproteins. Depending on the type of fatty acid attached to the protein, a proteolipid can often contain myristoyl, palmitoyl, or prenylgroups. These groups each serve different functions and have different preferences as to which amino acid residue they attach to. The processes are respectively named myristoylation, palmitoylation, and prenylation. Despite the seemingly specific names, N-myristoylation and S-palmitoylation can also involve some other fatty acids, most commonly in plants and viral proteolipids. The article on lipid-anchored proteins has more information on these canonical classes. There exist some rarer forms of protein acylation that may not have a membrane-related function. They include serine O-octanoylation in ghrelin, serine O-palmitoleoylation in Wnt proteins, and O-palmitoylation in histone H4 with LPCAT1. Hedgehog proteins are double-modified by palmitate and cholesterol. Some skin ceramides are proteolipids. The amino group on lysine can also be myristoylation via a poorly-understood mechanism.
In bacteria
All bacteria use proteolipids, sometimes confusingly referred to as bacterial lipoproteins, in their cell membrane. A common modification consists of N-acyl- and S‑diacylglycerol attached to an N-terminal cystine residue. Braun's lipoprotein, found in gram-negative bacteria, is a representative of this group. In addition, Mycobacterium O-mycolate proteins destinated for the outer membrane. The plant chloroplast is capable of many of the same modifications that bacteria perform to proteolipids. One database for such N-Acyl Diacyl Glycerylated cell wall proteolipids is DOLOP. Pathogenic spirochetes, including B. burgdorferi and T. pallidum, use their proteolipid adhesins to stick to victim cells. These proteins are also potent antigens, and are in fact the main immunogens of these two species. Products of nonribosomal peptide synthase may also involve a peptide structure linked to lipids. These are usually referred to as "lipopeptides".
