Apparent MW of protein on PAGE gel

[Russ_Lehrman]

> A fusion protein I expressed in E. coli has a apparent MW on SDS-PAGE gel ~5
> kdlager than it's calculated MW. This protein is 303 aa residues with
> acalculated MW of 35 kd but it runs at 40 kd position on SDS-PAGE. The
> proteinhas very high content of basic amino acids (32 lysines,  20 arginine
> and 16histidines). My question is : does high content of basic residues make
> aprotein run slower?I would apreciate very much any hints to my question.

Phosphorylation and other post-translational modifications are not typically
observed in E. coli.  Elution in a gel is not only a function of molecular
weight.  Often, reduction causes proteins to have apparently higher MW- even
in the presence of SDS.  You may be right to suspect the basicity of the
protein.  I can't address that from my experience but you may want to check
papers that discuss the characterization of histones (cytochrome c also). 
The latter proteins are all highly basic.   Russ