Palmitoylation of brain capillary proteins

Characterization of human palmitoyl-acyl transferase activity using peptides that mimic distinct palmitoylation motifs

The covalent attachment of palmitate to proteins commonly occurs on cysteine residues near either N-myristoylated glycine residues or C-terminal farnesylated cysteine residues. It therefore seems likely that multiple palmitoyl-acyl transferase (PAT) activities exist to recognize and modify these distinct palmitoylation motifs. To evaluate this possibility, two synthetic peptides representing these palmitoylation motifs, termed MyrGCK(NBD) and FarnCNRas(NBD), were used to characterize PAT activity under a variety of conditions. The human tumour cell lines MCF-7 and Hep-G2 each demonstrated high levels of PAT activity towards both peptides. In contrast, normal mouse fibroblasts (NIH/3T3 cells) demonstrated PAT activity towards the myristoylated substrate peptide but not the farnesylated peptide, while ras -transformed NIH/3T3 cells were able to palmitoylate the FarnCNRas(NBD) peptide. The kinetic parameters for PAT activity were determined using membranes from MCF-7 cells, and indicated that the K (m) values for palmitoyl-CoA were identical for PAT activity towards the two substrate peptides; however, the K (m) for MyrGCK(NBD) was 5-fold lower than the K (m) for FarnCNRas(NBD). PAT activity towards the two substrate peptides was dose-dependently inhibited by 2-bromopalmitate and 3-(1-oxo-hexadecyl)oxiranecarboxamide (16C; IC(50) values of approx. 4 and 1.3 microM, respectively); however, 2-bromopalmitate was found to be uncompetitive with respect to palmitoyl-CoA, whereas 16C was competitive. To seek additional evidence for multiple PATs, the effects of altering the assay conditions on the palmitoylation of MyrGCK(NBD) and FarnCNRas(NBD) were compared. PAT activity towards the two peptide substrates was modulated similarly by changing the ionic strength or incubation temperature, or by the addition of dithiothreitol. In contrast, the enzymic palmitoylation of the two peptides was differentially affected by N -ethylmaleimide and thermal denaturation. Overall, these data demonstrate that the enzymic palmitoylation of farnesyl- and myristoyl-containing peptide substrates can be differentiated, suggesting that multiple motif-specific PATs exist.

Posttranslational modification of tubulin by palmitoylation: I. In vivo and cell-free studies

It is well established that microtubules interact with intracellular membranes of eukaryotic cells. There is also evidence that tubulin, the major subunit of microtubules, associates directly with membranes. In many cases, this association between tubulin and membranes involves hydrophobic interactions. However, neither primary sequence nor known posttranslational modifications of tubulin can account for such an interaction. The goal of this study was to determine the molecular nature of hydrophobic interactions between tubulin and membranes. Specifically, I sought to identify a posttranslational modification of tubulin that is found in membrane proteins but not in cytoplasmic proteins. One such modification is the covalent attachment of the long chain fatty acid palmitate. The possibility that tubulin is a substrate for palmitoylation was investigated. First, I found that tubulin was palmitoylated in resting platelets and that the level of palmitoylation of tubulin decreased upon activation of platelets with thrombin. Second, to obtain quantities of palmitoylated tubulin required for protein structure analysis, a cell-free system for palmitoylation of tubulin was developed and characterized. The substrates for palmitoylation were nonpolymerized tubulin and tubulin in microtubules assembled with the slowly hydrolyzable GTP analogue guanylyl-(alpha, beta)-methylene-diphosphonate. However, tubulin in Taxol-assembled microtubules was not a substrate for palmitoylation. Likewise, palmitoylation of tubulin in the cell-free system was specifically inhibited by the antimicrotubule drugs Colcemid, podophyllotoxin, nocodazole, and vinblastine. These experiments identify a previously unknown posttranslational modification of tubulin that can account for at least one type of hydrophobic interaction with intracellular membranes.