Sensitive phosphoprotein detection in SDS-PAGE via Anthracene Chrome Red A stain

Protein phosphorylation, one of the most important post-translational modifications, plays critical roles in many biological processes. Thus, it is necessary to precisely detect, identify and understand the phosphoproteins from protein mixture for the study of cell biology. We introduce a sensitive and specific detection method for phosphoproteins in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Anthracene Chrome Red A (ACRA) combined with the trivalent metal ion (Al³⁺ ) is converted to fluorescent complex and the fluorescence is sharply increased by a change of pH environment. Phosphoproteins and non-phosphoproteins can be easily distinguished by the fluorescence quenching due to the structural change of ACRA-Al³⁺ -phosphoprotein complex, unlike non-phosphoprotein complex. The method using ACRA is a negative staining based on the fluorescence quenching and has a high sensitivity comparable to Pro-Q Diamond stain. ACRA stain can detect 1-2 ng of α-casein and β-casein, 8-16 ng of ovalbumin (OVA) and κ-casein within 130 min. Moreover, the ACRA stain showed similar linear dynamic ranges and RSD to Pro-Q stain. The linear dynamic ranges of ACRA and the values of correlation coefficient were for OVA (8-500 ng, correlation coefficient r = 0.999), α-casein (4-500 ng, r = 0.992), β-casein (4-500 ng, r = 0.996), and κ-casein (8-500 ng, 0.998), respectively. On the other hand, the values of the relative standard deviations (RSD) ranged from 2.33 to 3.56% for ACRA. The method is sensitive, specific, simple, rapid and compatible with total protein stain such as SYPRO Ruby stain. Therefore, ACRA stain can be an advanced method for phosphoprotein detection in gels.