Protein pI and migration on an SDS-PAGE gel

[Todd DeZwaan]

I am trying to in vitro translate a 33 kD protein using the Promega TnT
kit and Novagen's pCITE-2a vector.  I consistently see a 29 - 30 kD band
but not a 33 kD band.  My protein's predicted pI is 5.  Could this be
accounting for the discrepancy?  Does anyone know of a way to determine
how much smaller/larger a protein of a predicted MW will run on an
SDS-PAGE gel based on its pI?  Thanks in advance.

Todd