Phosphorylation-regulated crosslinking of gold nanoparticles: a new strategy for colorimetric detection of protein kinase activity

Accurate and rapid detection of protein kinase activities is of great significance because protein kinases play important regulatory roles in many vital biological processes. Herein, we wish to report a facile colorimetric protein kinase assay based on the phosphorylation-tuned crosslinking of gold nanoparticles (GNPs) by using protein kinase A (PKA) as a proof-of-concept target. In this new strategy, a biotinylated peptide (biotin-LRRASLG) is used as the PKA-specific substrate. When mixed with streptavidin-functionalized GNPs (STV-GNPs), the positively charged biotin-peptide will combine with different GNPs both through the specific STV–biotin binding and through electrostatic interactions, which will lead to the crosslinking and coagulation of GNPs. In contrast, under the catalysis of PKA, the biotin-peptide will be phosphorylated at the serine residue and its net charge will be obviously altered, which may significantly weaken the electrostatic interaction between the phosphopeptide and GNPs and thus effectively prevent the STV-GNPs from crosslinking and settlement. Therefore, by viewing the color changes of the GNPs, the PKA activity can be easily detected by the naked eye.