phosphorylted protein smaller not bigger? - (May/10/2011 )

Hi
I am researching and re researching Sirt1 deacetylase
It appears that the protein, when phosphorylated, at least for mouse, runs further on SDS gel. can anyone shed light on this?
thanks
Suse

the incorporated phosphate(s) can increase mobility, it increases negative charge on the protein. we used urea gels to visualize phosphorylated vs unphosphorylated proteins and the phosphorylated ran faster (urea enhances charge separations).