acidic proteins in SDS-PAGE - (Sep/30/2006 )

Hi!
I have protein which is concidered to be acidic
(pI = 3.9) when i do SDS-PAGE i get a size of 20 kDA
instead of 12 kDa. and i don't think it's the dimer form.
are their any actions i can do in order to see the real
size? or how can i be sure it's not a dimer?

p.s
This protein has no cysteins, so increasing DTT won't help...

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Is it a glycoprotein?

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it's not a glycoprotein. it's a protein who's involved adhesive function

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I would think of acetylations, phosphorylations, sulfonation (all acidic) or methylation as posttranslational modifications;

acetylations are known for histones (as epigenetic regulation method), transcription factors, receptors and others;

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