Dear Dave,
We have two observations that relate to your question. First, when we
photoaffinity label our protein (activase) with azidoATP we sometimes
observe larger than expect decreases (i.e., 2 kD) in electrophoretic
mobility. I say sometimes because it depends entirely on which site on
the protein is modified (for ref. see Biochemistry 33, 14879-14786
[1994]). Second, when we modified a tryptophan in one of the sites using
directed mutagenesis, there was a slight decrease in electrophoretic
mobility on SDS-PAGE when Cys was substituted for Trp (for ref. see
Biochemistry 35, 8143-8148 [1996]). Our evidence suggests that changes in
this region of the polypeptide affect the interaction of SDS with the
polypeptide. We have looked at this experimentally by varying the amount
of SDS in the gel. Sure enough, the changes in electrophoretic mobility
are much smaller when there is more SDS in the gel.
Good luck........MIKE SALVUCCI, USDA-ARS, PHOENIX, AZ
