gamma-glutamyltransferase and its isoform mediate an endoplasmic reticulumstress response

Although use of multiple alternative first exons generates unique noncoding 5'-ends for gamma -glutamyltransferase (GGT) cDNAs in several species, we show here that alternative splicing events also alter coding exons in mouse GGT to produce at least four protein isoforms, GGT Delta1 introduces CAG f our bases upstream of the primary ATG codon and encodes an active GGT heter odimeric ectoenzyme identical to constitutive GGT cDNA but translational ef ficiency is reduced 2-fold, GGT Delta2-5 deletes the last eight nucleotides of exon 2 through most of exon 5 in-frame, selectively eliminating residue s 96-231 from the amphipathic N-terminal subunit, including four N-glycan c onsensus sites, while leaving the C-terminal hydrophilic subunit intact. GG T Delta7 introduces 22 bases from intron 7 causing a frameshift and a prema ture stop codon so a truncated polypeptide is encoded terminating with 14 n ovel residues but retaining the first 339 residues of the native GGT protei n. GGT Delta8-9 deletes the terminal four nucleotides of exon 8 plus all of exon 9 and inserts 24 bases from intron 9 in-frame so the C-terminal subun it of the encoded polypeptide loses residues 401-444 but gains eight intern al hydrophobic residues. In contrast to the product of GGT Delta1, those de rived from GGT Delta2-5, Delta7, Delta8-9 all lack transferase activity and persist as single-chain glycoproteins retained largely in the endoplasmic reticulum as determined by immunofluorescence microscopy and constitutive e ndoglycosidase Il sensitivity in metabolically labeled cells. The developme ntal-stage plus tissue-specific regulation of the alternative splicing even ts at GGT Delta7 and GGT Delta8-9 implies unique roles for these GGT protei n isoforms, The ability of the GGT Delta1 and GGT Delta7 to mediate the ind uction of C/EBP homologous protein-10, CHOP-10, and immunoglobulin heavy ch ain binding protein, BiP, implicates a specific role for these two GGT prot ein isoforms in the endoplasmic reticulum stress response.