Isolation of Endoplasmic Reticulum and Its Membrane.

The association of ribosomes with the rough endoplasmic reticulum (ER) is dependent on Mg. The ribosomes can be stripped from the ER by removal of Mg from the medium, resulting in a reduction in the ER membrane density and a diagnostic shift in migration when ER vesicles are analyzed by equilibrium density gradient centrifugation. Here, I describe the isolation of microsomes from Arabidopsis, followed by the use of the density shift approach in conjunction with equilibrium density gradient centrifugation as a means to diagnose whether a protein is associated with the ER. The same approach can also be used as a means to enrich for ER membranes.