Why does Bmi1 migrate as a doublet by western blot?

Bmi1 undergoes numerous post-translational modifications and can be heavily phosphorylated, as described on the PhosphoSite.org protein page (https://www.phosphosite.org/proteinAction.action?id=1286622&;showAllSites=true). Therefore, we believe the upper band represents phosphorylated Bmi1.

Multiple papers have also described Bmi1 migrating as a doublet by western blot, as shown in the examples below.

• Gunster, et. al [see Mol. Cell Biol. (1997) (4), 2326-35, PMID: 9121482 https://www.ncbi.nlm.nih.gov/pubmed/9121482] characterizes Bmi1 as migrating as a doublet, as seen in U2-OS cells (Fig. 5C, input lane). These bands migrate at a larger size compared to the predicted weight.
• Mustafi, et. al [see Mol. Cancer (2017) 16:56, PMID: 28270146 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5341428/] shows Bmi1 (Fig. 1a) migrating as a doublet where the upper band is reduced with lambda-phosphatase treatment, adding evidence to the theory that the upper band may be phosphorylated Bmi1. 

Last updated: February 27, 2024