A novel method for the production of fully modified K-Ras 4B

Post-translational modifications in proteins play a major functional role. Post-translational modifications affect the way proteins interact with each other, bind nucleotides, and localize in cellular compartments. Given the importance of post-translational modifications in protein biology, development of methods to produce post-translationally modified proteins for biochemical and biophysical studies is timely and significant. At the same time, obtaining post-translationally modified proteins in bacterial expression systems is often problematic. Here, we describe a novel recombinant approach to prepare human K-Ras 4B, a protein that is post-translationally farnesylated, proteolytically cleaved, and methylated in its C-terminus. K-Ras 4B is a member of the Ras subfamily of small GTPases and is of interest because it is frequently mutated in human cancer. The method relies on separate production of two structural domains-the N-terminal catalytic domain and the C-terminal peptide chemically modified with S-farnesyl-L-cysteine methyl ester. After the two domains are prepared, they are ligated together using the transpeptidase enzyme, sortase. Our procedure starts with the use of the plasmid of K-Ras 4B catalytic domain containing the sortase recognition sequence. After this, we describe the bacterial expression and purification steps used to purify K-Ras 4B and the preparation of the conjugated C-terminal peptide. The procedure ends with the sortase-mediated ligation technique. The produced post-translationally modified K-Ras 4B is active in a number of assays, including a GTP hydrolysis assay, Raf-1 binding assay, and surface plasmon resonance-based phospholipid binding assay.