Structural domains of apolipoprotein(a) and its interaction with apolipoprotein B-100 in the lipoprotein(a) particle.

The structural domains of human apolipoprotein(a) [apo(a)] and its interaction with apolipoprotein B-100 (apo B-100) in the lipoprotein(a) [Lp(a)] particle were investigated by limited proteolysis with thermolysin and cathepsin D. We characterized the proteolytic products by sodium dodecyl sulfate-polyacrylamide gradient gel electrophoresis, followed by immunoblotting using different antibodies. For apo B-100 in Lp(a), the digestion patterns were found to be identical to those previously described [Chen et al. (1989) J. Biol. Chem. 264, 14369-14375; Chen et al. (1991) J. Biol. Chem. 266, 12581-12587] for apo B-100 in LDL. Thus, we compared the digestion patterns of apo B-100 in Lp(a) resolved under reducing and nonreducing migrating conditions. Using an antibody specific for a synthetic peptide of apo B-100 (residues 4004-4021), we confirmed that apo B-100 was linked to apo(a) by its C-terminal end. Various Lp(a)s isolated from several donors, and containing different isoforms, were used to study the structural domains of apo(a). Using the same procedure as for apo B-100, several common features were found for the different isoforms. (1) Apo(a) can be cleaved into two structural domains: one was of constant size (170 kDa) and was linked to apo B-100. Using an antibody specifically directed against kringle V, we demonstrated that this fragment corresponded to the C-terminal part of apo(a). (2) The other domain, whose size varied according to the digested apo(a) isoform, was not linked to apo B-100.(ABSTRACT TRUNCATED AT 250 WORDS)