Effects of membrane washes on association of MP with ER. Microsomal membranes were collected as in Fig. 4 and incubated 30 min on ice with 2.5 M urea, 0.5 M NaCl, 1% Triton X-100, or 1% CHAPS. After samples were collected by centrifugation, they were subjected to Western blot analysis with anti-MP, anti-GFP, anti-ATPase, and anti-BiP antibodies.