A polypeptide homologous to human and mouse J chain has been identified in the high molecular weight (HMW) Ig of the bullfrog, Rana catesbeiana. In previous studies, we had detected a component that was similar in size to mammalian J chains and that, relative to L chains, migrated rapidly to the anode in alkaline-urea PAGE; however, its mobility was less than that of mammalian J chains. We now demonstrate that this component is covalently linked to the H chain of R. catesbeiana HMW Ig. All of the disulfide bridges of this polypeptide, like those of human and mouse J chain, can be cleaved by reducing agents even in the absence of denaturing solvents. The putative frog J chain was isolated by a procedure that did not require preliminary purification of the HMW Ig. The chain differed in amino acid composition from L chains but resembled J chains from several other species. Tryptic peptides were isolated and sequenced. Except for a single heptapeptide, the peptides could be aligned by virtue of their similarity to segments of human and mouse J chain. Of the 116 residues that were placed, 55 were identical with residues in human J chain and 60 with residues in mouse J chain. The six cysteine residues identified in the frog J chain are at the same positions as six of the eight cysteines in the human and mouse J chains. The results indicate significant conservation in structure between amphibian and mammalian Ig J chains.

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