Evolutionary origin of the T lymphocyte receptor—I: immunochemical investigation of 1mmunoglobulin-like cell surface protein of carp thymocyte

Abstract The mIg of carp thymocytes has been analysed by lactoperoxidase-catalysed membrane iodination, sandwich precipitation of the solubilized membrane proteins, and SDS-PAGE. ∗ The thymocytic mIg is bound by all antisera with activity against carp IgM, Fc, Fab, Fd and μ chain but not by anti-L chain serum. An influence of carbohydrate determinants could be excluded. The thymocytic mlg contains determinants cross-reactive with humoral IgM and the B cell mIg, a monomeric IgM. It consists of two chains with 65,000 d molecular weight, covalently and non-covalently linked. A 260,000 d molecule, detectable in the non-reduced membrane lysate, is a non-covalently bound dimer of the 130,000 d unit. It may be an artifact of the solubilization procedure. The isolated thymocyte mIg is rapidly degraded by proteases; in particular, the 65,000 d chain is split into 45,000 and 20,000 d portions. This degradation can be inhibited by PMSF and low temperature. The antigenic similarity of the thymocytic 65,000 d chain and the μ chain refers to a common origin of both structural c genes. But according to the existing differences already at the level of bony fishes, the thymocyte Ig chain represents a different class and should be designated τ chain.