Protein palmitoylation in membrane trafficking

Introduction Protein palmitoylation was first described more than 20 years ago for the myelin proteolipid protein, but the functional significance of this post-translational modification remains obscure. Progress has been hampered by the lack of specific inhibitors for acylation and because of the difficulty of developing an assay to isolate the palmitoylating enzyme(s). Palmitoylation generally occurs through labile thioester bonds and it is the only lipid modification that is readily reversible. Palmitoylation may determine the extent to which a protein is associated with membranes, but evidence from a number of groups is beginning to suggest that the role of covalent fatty acids is more complex than simply serving as a hydrophobic membrane anchor and suggest instead that it may function to assemble protein-protein complexes at or within the membrane. These interactions could then be modulated by cycles of acylation-deacylation. Although not as common as other reversible modifications of proteins, such as phosphorylation, reversible fatty acylation might be expected to have equally important regulatory functions. Click and Rothman [ 11 first suggested a role for fatty acylation in membrane trafficking by demonstrating that vesicular transport between the Cis and medial stacks of the Colgi apparatus is stimulated by palmitoyl-CoA in a cell-free transport assay. Further studies showed that fatty-acyl-CoA is required both for the budding [2,3] and fusion [4] of transport vesicles. The budding of Golgi-coated vesicles was recently reconstituted by using partly purified membranes and these results suggest that palmitoyl-CoA is required for fission of the vesicle from the membrane [3]. T o identify an acylated protein that might act as a palmitate donor or acceptor during vesicular transport, we examined protein acylation under conditions where transport is inhibited. It is well recognized that vesicular transport ceases as cells undergo mitosis [5]. We showed that a protein with an apparent molecular