Problem with protein precipitation - (Mar/17/2009 )

Hi everybody,
I have a big problem : I am trying to precipitate proteins from nuclear and cytoplasmic fractions using acetone.
I have unfortunately a problem since I have got a loss of 80 % of my proteins after precipitation. Moreover, the pellet of protein is grey whereas it should be white... Do you know what could be the problem ?
I use a buffer with HEPES 10 mM, MgCl2 1.5 mM, KCl 10 mM, DTT, protease and phosphatase inhibitors and 0.05 % NP40... I don't know what to do...
Thanks a lot for your advices.

maybe you should try precipitating with ammonium sulphate................