The medial Golgi enzymes, N-acetylglucosaminyltransferase I (NAGT I) a
nd mannosidase II (Mann II), and the trans Golgi enzyme, beta-1,4-gala
ctosyltransferase (GalT) were each retained in the endoplasmic reticul
um (ER) by grafting on the cytoplasmic tail of the p33 invariant chain
. Transient and stable expression of p33/NAGT I in HeLa cells caused r
elocation of endogenous Mann II to the ER and transient expression of
p33/Mann II had a similar effect on endogenous NAGT I. Neither of thes
e endogenous medial enzymes were affected by transient expression of p
33/GalT. These data provide strong evidence for kin recognition betwee
n medial Golgi enzymes and suggest a role for them in the organization
of the Golgi stack.