Figure 2.

The purified membranes are highly enriched for a Golgi marker enzyme and deenriched for marker proteins of the ER, lysosomes, endosomes, and plasma membrane. Golgi-enriched membranes and the crude microsomes from which they were purified were analyzed by quantitative immunoblotting, as described in MATERIALS AND METHODS. Compared with the microsomes, the purified membranes were enriched ∼44-fold for the Golgi enzyme galactosyltransferase and were deenriched by ∼50% for the ER luminal protein BiP. Both the mature form of cathepsin D, a lysosomal enzyme, and the transferrin receptor, which is a marker for endosomes and the plasma membrane, were readily detectable in the microsomes but could not be detected in the purified membrane fraction.