Cell lysis method to get "intact" nuclear protein complex for CO-IP? - (May/13/2015 )

I would like to find cell lysis method to solubilize/break-up nuclear membrane without breaking/denaturing protein-protein interaction of protein complex in nucleus before doing CO-IP. I need to find method to breakup nuclear membrane, however It seems that RIPA buffer and sonication method are not recommend which can denature protein-protein interaction. Could you recommend me a method to break up nuclear membrane to get "intact" nuclear protein complex for CO-IP? especially chemical method.

Thank you so much

I use a buffer called IPPT (based on this paper) for RB.