POSTTRANSLATIONAL PROCESSING OF THE PHOSPHATIDYLSERINE DECARBOXYLASE GENE-PRODUCT IN CHINESE-HAMSTER OVARY CELLS

We have isolated a full-length cDNA clone of the Chinese hamster ovary (CHO) pssC gene, which encodes mitochondrial phosphatidylserine decar boxylase. The cDNA clone is capable of increasing phosphatidylserine d ecarboxylase activity to Ii-fold in CHO-KI cells. The pssC gene produc t predicted from the cDNA sequence is composed of 409 amino acid resid ues. In an in vitro translation system coupled with in vitro transcrip tion, the cDNA clone directs the formation of a protein with an appare nt molecular mass of 46 kDa. In CHO-K1 cells, the cDNA clone leads to the production of two major peptides with apparent molecular masses of 38 and 34 kDa, as determined by Western blotting with an antibody rai sed against a recombinant pssC protein. When CHO-K1 cells transfected with the cDNA clone are labelled with [S-35]methionine for a short per iod, proteins immunoprecipitated with the antibody lack radioactive 38 and 34 kDa peptides, but contain two radioactive peptides with appare nt molecular masses of 46 and 42 kDa instead. The pssC gene product pr edicted from the cDNA sequence has, near its C-terminus, a unique Leu- Gly-Ser-Thr sequence which is known as a processing site for Escherich ia coli phosphatidylserine decarboxylase. A mutant pssC cDNA clone, in which Ser(378) in the conserved sequence is replaced by Ala, leads to overproduction of 46, 42 and 38 kDa peptides, but not a 34 kDa peptid e. This mutant clone is incapable of increasing phosphatidylserine dec arboxylase activity, in contrast to the wild-type clone. These results indicate that the processing at the Leu-Gly-Ser-Thr sequence is essen tial for formation of the active enzyme. Thus, the pssC gene product i s converted into mature phosphatidylserine decarboxylase through multi ple steps of post-translational processing.