γ-Glutamyltransferase and Its Isoform Mediate an Endoplasmic Reticulum Stress Response

Abstract Although use of multiple alternative first exons generates unique noncoding 5′-ends for γ-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. GGTΔ1 introduces CAG four bases upstream of the primary ATG codon and encodes an active GGT heterodimeric ectoenzyme identical to constitutive GGT cDNA but translational efficiency is reduced 2-fold. GGTΔ2–5 deletes the last eight nucleotides of exon 2 through most of exon 5 in-frame, selectively eliminating residues 96–231 from the amphipathic N-terminal subunit, including four N-glycan consensus sites, while leaving the C-terminal hydrophilic subunit intact. GGTΔ7 introduces 22 bases from intron 7 causing a frameshift and a premature stop codon so a truncated polypeptide is encoded terminating with 14 novel residues but retaining the first 339 residues of the native GGT protein. GGTΔ8–9 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 subunit of the encoded polypeptide loses residues 401–444 but gains eight internal hydrophobic residues. In contrast to the product of GGTΔ1, those derived from GGTΔ2–5, Δ7, Δ8–9 all lack transferase activity and persist as single-chain glycoproteins retained largely in the endoplasmic reticulum as determined by immunofluorescence microscopy and constitutive endoglycosidase H sensitivity in metabolically labeled cells. The developmental-stage plus tissue-specific regulation of the alternative splicing events at GGTΔ7 and GGTΔ8–9 implies unique roles for these GGT protein isoforms. The ability of the GGTΔ1 and GGTΔ7 to mediate the induction of C/EBP homologous protein-10, CHOP-10, and immunoglobulin heavy chain binding protein, BiP, implicates a specific role for these two GGT protein isoforms in the endoplasmic reticulum stress response.