Hi Carlo, I have to extract a membrane protein (together with a cytoplasmatic one subjected to degradation after homogeneization) from muscle biopsy and I suggested you, at least, 3 buffers; with your cells samples it will be not difficult made test and you can choose depending to your reagents and to your protein Assay Kit

1 RIPA BUFFER: 50mM Tris pH 7,5, 150mM NaCl, 1% Triton, 0,5% Sodium deoxycholate, 0,1% SDS + protease inhibitors cocktail (there are a lot of of RIPA recipe, so you can also find variation on web)

2 SDS-UREA: 125mM Tris pH6,8 + 2%SDS + 4M Urea without protease inhibitors (with 4M or more Urea inhibitors is not needed, if you prefer you can add it, in my hand work very well without them also with the "tough" degraded protein. I made 8,5M Urea stocks, it is very ease to dissolve, and freezed them at -20°C. Do not refreezed the urea be left, wasted it)

3 SDS_EDTA: 125mM Tris pH6,8, 4%SDS + 10mM EDTA + protease inhibitors cocktail (I am a little paranois, never used Urea before try newone extrapolated from 5 articles, yes I am paranoic!)

For protein assay kit I have problem with bradford and sds, but with no more that 4%SDS,  and a cuvette of 1ml I don't have preoblems; BCA forever anyway!!!

For the omogeneization protocol I have a stirrer and a sonicator in my lab (fortunately, with the muscle can be difficult the omogenation) so I added the cold buffer omogenized and sonicated always on ice and centrifuged for 10 min at 12000rpm at 4°C, dosage with kit, add laemmli and boiled at 95°C 5min

Goooood luck!