Partial primary structure of human pregnancy zone protein: extensive sequence homology with human alpha 2-macroglobulin.

Abstract

Human pregnancy zone protein (PZP) is a major pregnancy-associated protein. Its quaternary structure (2 covalently bound 180-kDa [kilodalton] subunits, which are further non-covalently assembled into a tetramer of 720 kDa) is similar to that of human .alpha.2-macroglobulin (.alpha.2M). Complete or partial sequence determination of a random selection of 38 tryptic peptides covering 685 residues of the subunit of PZP showed that PZP and .alpha.2M indeed are extensively homologous. In the stretches of PZP sequenced so far, the degree of identically placed residues in the 2 proteins is 68%, indicating a close evolutionary relationship between PZP and .alpha.2M. Although the function of PZP in pregnancy is largely unknown, its close structural relationship to .alpha.2M suggests analogous proteinase binding properties and a potential for being taken up in cells by receptor-mediated endocytosis. In this regard these studies indicate a bait region in PZP significantly different from that present in .alpha.2M. PZP could be the human equivalent of the acute-phase .alpha.-macroglobulins (e.g., rat .alpha.2M and rabbit .alpha.1M) described earlier.