Aggregation behavior of the bovine beta-crystallin Bp chain studied by limited proteolysis

The bovine beta-crystallin Bp chain is organized into two very similar domains, with short extensions at both N- and C-termini, and two alternative models for the beta Bp dimer have been proposed (Wistow, G., Slingsby, C., Blundell, T., Driessen, H.P.C., De Jong, W.W. and Bloemendal, H. (1981) FEBS Lett. 133, 9-16). By limited proteolysis the C-terminal arms can be cleaved off rapidly from the beta Bp dimer, while the N-terminal arms are more difficult to remove. Trypsin divides the beta Bp chain into two fragments which approximately correspond to the two structural domains. Dissociation and reassociation of the different products of limited proteolysis indicated that: the C-terminal arm extends freely from the surface and is not involved in subunit-contact; at least one N-terminal arm seems required for dimer formation; the N-terminal domains have a greater tendency to associate than the C-terminal domains and, when mixed, the purified domains reassociate partially to a Mr 50 000 structure like native beta Bp. These findings support the more extended dimer model of beta Bp.