Comparison of conformational changes of pregnancy zone protein and human alpha 2-macroglobulin, a study using hydrophobic affinity partitioning.

Conformational changes of human alpha 2-macroglobulin (alpha 2M) and pregnancy zone protein (PZP), reflected in changes in surface hydrophobicity, have been studied. The results show that the conformation of alpha 2M is governed by the degree of 'trapping'. Thus, cleavage in the bait region and of the thiol ester by proteinase treatment causes a two-fold increase in surface hydrophobicity of alpha 2M. However, the increase is still higher (three-fold) when the thiol esters in alpha 2M alone are cleaved by methylamine. Cyanylation of the thiol groups exposed upon methylamine treatment yields a derivative with the same hydrophobicity as native alpha 2M. Treatment of this derivative with chymotrypsin restores the hydrophobicity to that of methylamine-treated alpha 2M. Since the C-terminal 18 kDa fragment of alpha 2M exhibits no hydrophobicity, the change in hydrophobicity seems not to reside in the receptor binding site. In contrast to alpha 2M, modification of both native and methylamine-treated PZP with chymotrypsin gives a reduction (about 40%) in hydrophobicity. The change in hydrophobicity is insignificant on treatment with methylamine alone. Furthermore, hydrophobic interactions appear not to contribute to tetramerization of PZP. The present study indicates major differences in the conformational states of alpha 2M and PZP as reflected in the hydrophobic surfaces exhibited.