Compartment a t10n of pros fanoid biosynthet1c enzymes

Mammalian cells contain two isozvmes of PGH synthase (PGHS), both of which catalyze the conversion of arachidomc to PGH; To determine the subcellular distribution of PGHS-1 and -2, we used immunocytochemica! protocols to detect the proteins and histochemical assays to measure the activities Using both light microscopy following differential permeabilization ut'cells with detergents, and. more lecentlv. immunoelectron microscopy, we ha\c found that both isoz\mes arc present on the lumenal surfaces of the ER and N'E and that PGHS-1 and PGHS-2 iminunoreactivity is uniformly disiiiinited on the lumenal surfaces ofboth the inner and outer membranes of the nucleai envelope However, Wesiein transfer blotting of subcellular fractions indicate thai 1'dllS-l is equally concentrated in the endoplasmic reticiilum (ER) and nuclear envelope ( \'E) fractions, whereas PGHS-2 is more concentrated in the NE than in the ER Recent studies have established that the ('-terminal -I'.'STEI. sequences of both PGHS-1 and PGHS-2 serve as ER retention signals and that deletion uf the unique C-terminal 18 amino acid cassette of PGHS-2 does not atl'ect the subcellular distribution of this isozyme Work is in progiess to determine the basis for the unequal distribution of PGHS-: in the NE versu-. the ER.