MULTIPLE ISOFORMS OF METALLOTHIONEIN ARE EXPRESSED IN THE PORCINE LIVER

Isogenes are highly homologous to each other and are often difficult t o ascertain, as has been the case with metallothionein, a metal-bindin g protein rich in cysteines. Conventional separation of metallothionei n isoforms relied on ion exchange chromatography of the proteins, or s creening for the sequences from gene libraries. In this study, a combi nation of RT-PCR and partial protein sequencing is used in the identif ication of metallothionein isogenes expressed in porcine liver. By thi s approach, we have identified expressed coding sequences which consti tute 10 new isogenes. Of the four known groups of metallothioneins (MT ), phylogenetic analyses place these pig isogenes in the MT-1 group, e xcept two which are identified as being closely related to MT-2, and n one in groups 3 and 4. The isogenes are thus named pMT-1a to -1g, and pMT-2a and -2b. While each of the isogene sequences is unique, two iso genes, pMT-1e1 and pMT-1e2, share an identical amino acid sequence, di ffering only in specific codons. Two others, pMT-1b and pMT-1g, have a cysteine substituted by arginine, the first such sequence ever detect ed in MT.pMT-2a and pMT-2b are closely aligned with the MT-2 group of vertebrates, in spite of the absence of a characteristic acidic amino acid at position 10 or 11, common in other mammalian metallothioneins. (C) 1998 Elsevier Science B.V. All rights reserved.