Comparative Strategies for Ubiquitination Detection in Mammalian Cell Lysates Using SMAD2/SMURF2 as a Model

Ubiquitination, a post-translational modification, is a critical regulator of intracellular protein function. Ubiquitination modulates protein functions by promoting proteasomal degradation or altering subcellular localization through ubiquitin chain-dependent signaling. Here, we describe two cell lysis methods for detecting SMAD2 ubiquitination levels in HEK293T cells and compare their effectiveness in analyzing protein ubiquitination levels. Protein overexpression in cells was induced by transient transfection. The plasmids (HA-Ub, FLAG-SMAD2, and MYC-SMURF2) and transfection reagents were separately added to basal medium, mixed, and the mixture was added to the cells. Prior to harvest, MG132 was added to inhibit proteasomal degradation and enhance ubiquitinated protein accumulation. The primary divergence between the two experimental approaches is their cell lysis methods-the ice-bath method (performed at 4 °C) and the heat-treatment method (involving incubation at 95 °C), which substantially affects the efficiency of protein lysis. After cell lysis was completed, the cell lysate, agarose beads, and FLAG antibody were mixed and incubated at 4 °C overnight. Ubiquitinated proteins were then detected by western blot analysis. Before detecting ubiquitinated proteins, a light-chain antibody was used for secondary antibody incubation. Then, ubiquitination bands were detected. The results show that both the ice-bath method and the heat-treatment method can be used to detect ubiquitination levels, while the heat-treatment method may make it easier to detect ubiquitination of SMAD2. This study delineates and compares two cell lysis methods for measuring ubiquitination levels in mammalian cells, using SMURF2/SMAD2 as a model, to assist researchers in selecting more appropriate methods for detecting the ubiquitination levels of substrate proteins.