What cause "downshift" in SDS-PAGE? - (Jan/09/2011 )

There's a protein sample. You splite it into two and subject them to two treatment (presumably 1 control treatment and 1 testing treatment). After that you denature it and run SDS-PAGE. You find that the testing treatment "downshift" the protein, i.e. protein migrate faster than the control. What is/are the possible causes of this?

Everybody knows the most obvious one is phosphatase (or dephosphrylation). I just wanna if there's any possbile mechanism to cause this observation.

Actually it´s phosphorylation, not dephosphorylation that causes proteins to migrate faster.

ElHo on Mon Jan 10 09:35:40 2011 said:

not to my understanding. Phosphorylation should cause a upshifted band.

deglycosylation, demethylation, delipidation...

phosphorylation will add very little to molecular weight but will increase negative charge on molecule (even in the presence of sds). you can enhance the downshift caused by phosphorylation by running the protein on urea-page.

I've actually seen both and, like everything else in science, there is no 100% rule. Some proteins will upshift or run slower with the phosphate while others will downshift or run faster with the phosphate. Generally it is thought that a phosphorylated protein will run slower and be the upshifted band but again, not always. For example, phosphorylated MCM2 runs faster and results in a down-shifted band while phosphorylated cyclin B runs slower.

what is the testing treatment???