Prostaglandin endoperoxide H (PGH) synthases 1 and 2 are both 
membrane-associated proteins localized to the endoplasmic reticulum (ER) and 
nuclear envelope. The carboxyl terminal tetrapeptides of PGH synthases 1 and 2 
are of the form -P/STEL. These sequences are similar to the -KDEL retention 
signal sequence characteristic of many proteins localized to the ER. To 
determine if the -PTEL sequence (residues 597-600) functions as an ER retention 
signal for ovine PGH synthase-1, we prepared and analyzed five mutants (L600N, 
L600R, L600V, E599Q, and delta 597), all having modifications that would be 
expected to alter the subcellular location of PGH synthase-1 if the -PTEL 
sequence were involved in ER targeting. Native ovine PGH synthase-1 and each of 
the five mutants were subcloned into the pSVT7 expression vector and were 
expressed transiently in cos-1 cells. The L600N, L600R, E599Q, and delta 597 
mutants retained both cyclooxygenase and peroxidase activities. Moreover, when 
subjected to immunocytofluorescent staining, cos-1 cells expressing native and 
mutant enzymes showed similar patterns of fluorescence corresponding to ER and 
nuclear envelope localization. Finally, culture media bathing cos-1 cells 
transfected with native or mutant PGH synthases were tested for secreted PGH 
synthase-1 protein by Western blotting, but no PGH synthase-1 was detected in 
any of the culture media. Our results demonstrate that mutations in the 
C-terminal sequence-PTEL do not change the subcellular location of ovine PGH 
synthase-1. Thus, targeting of PGH synthase-1 to the ER can occur independent of 
its -PTEL sequence.