Modification of α-crystallin subunit chains and formation of α-neoprotein molecules: Role of SH groups

Abstract

Immunoadsorbents with bound antibodies restricted to determinants dependent on α-crystallin's quaternary structure permitted the fractionation of the population of 125I-labeled α-crystallin molecules, treated by iodoacetic acid, into molecules in which the native structure was still preserved and molecules with a completely different quaternary structure than the native protein. Parallel experiments with [ 14C]iodoacetic acid yielded information on the percentage of blocked SH groups in each of the above two fractions. The presence of molecules formed by A with B-chain association was established by sequential binding first to an immunoadsorbent with antibodies restricted to determinants located on α-crystallin's A-subunit chains as ligand and second, after desorption, to an immunoadsorbent with antibodies to B chains as ligand. With the aid of these techniques, it was established that (i) The modified α-crystallin molecules with quaternary determinants of the native protein contained a maximum of 23% blocked SH groups, indicating that the carboxymethylation involved only the fastreacting surface SH groups. (ii) The modified α-crystallin molecules without the native protein's quaternary structure were built by a different association between A and B subunits than in α-crystallin, indicating formation of α-neoprotein molecules. (iii) Monomeric A chains with all SH groups carboxymethylated, and monomeric B chains in a ratio of 1A:5B, 2A:1B, and 5A:1B in urea solution, associate on dialysis, forming α-neoprotein molecules.