Rat liver Golgi stacks were extracted with Triton X-100 at neutral pH.
After centrifugation the low speed pellet contained two medial-Golgi
enzymes, N-acetylglucosaminyltransferase I and mannosidase II, but no
enzymes or markers from other parts of the Golgi apparatus. Both were
present in the same structures which appeared, by electron microscopy,
to be small remnants of cisternal membranes. The enzymes could be rem
oved by treatment with low salt, leaving behind a salt pellet, which w
e term the matrix. Removal of salt caused specific re-binding of both
enzymes to the matrix, with an apparent dissociation constant of 3 nM
for mannosidase II. Re-binding was abolished by pretreatment of intact
Golgi stacks with proteinase K, suggesting that the matrix was presen
t between the cisternae.