The physical association between rat liver mitochondria and rough endoplasmic reticulum : I. Isolation, electron microscopic examination and sedimentation equilibrium centrifugation analyses of rough endoplasmic reticulum-mitochondrial complexes

Rough endoplasmic reticulum-mitochondrial (RER-MT) complexes have been isolated from rat liver homogenates by rate zonal Centrifugation using a reorienting zonal rotor. Electron microscopic examination of the isolated complexes reveals a close association between rough endoplasmic reticulum (RER) and mitochondria. The associated RER appears as bilamellar sheets as it does in intact liver tissue, not as microsomal vesicles. When the complexes are subjected to sedimentation equilibrium Centrifugation, the marker enzymes for mitochondria and RER coband at an equilibrium density of 1.190. Electron microscopic analysis of the complexes after sedimentation equilibrium Centrifugation again reveals a close association between RER and mitochondria. Treatment of the complexes with 500 mM KCl or 500 mM KCl plus 20 mM EDTA resulted in a shift in the equilibrium density of the complexes to 1.180 and 1.176, respectively. Concomitant with the density shift was a release of A₂₆₀ units to the top of the gradient. After incubating KCl-EDTA stripped complexes with cytoplasmic ribosomes and ribosomal subunits, the complexes band at the same equilibrium density, 1,190, as do untreated complexes. In order to completely remove the associated RER it is necessary to treat the complexes with digitonin at a concentration of 0.13 mg digitonin/mg protein. Our data suggest that a fraction of the total cellular RER is physically associated with rat liver mitochondria.