which amino acid mimicking phosphotyrosine?

[hammer at liai.org]

Hi netters!

I would like to make a constitutive active mutant of a protein which 
is activared by tyrosine phosphorylation.

I am thinking of substituting the the tyrosine residue by aspartic 
acid. Is aspartic acid mimicking phosphotyrosine?

Are there any references?

Any hint will be appreciated,

thanks,

Stephan