The 84-amino-acid-long sequence of perlustrin showed homology of the abalone nacre protein to the N-terminal domain of mammalian insulin-like growth factor binding proteins (IGFBPs), Despite the evolutionary distance between mollusks and mammals, the sequence identity was 40% including 12 conserved cysteines. However, the residues which were suggested recently to bind IGF-II in a complex with IGFBP-5 were conserved only partially. Nevertheless, perlustrin bound human IGFs with K-D similar to 10(-7) M. This was the same affinity range as measured before for the interaction of isolated IGFBP-5 N-terminal domains with IGFs. Moreover, perlustrin bound bovine insulin with only approximately two- to sevenfold lower affinity than IGFs, Sequence similarity and growth factor binding identified perlustrin unequivocally as a member of the IGFBP family, the first found in an invertebrate biomineral, Nacre is known to contain proteinaceous factors which promote bone formation in vitro and in vivo. Bone contains IGFBPs which influence bone metabolism in many ways by modulating either IGF effects or IGF independently. Thus, perlustrin may provide a first clue at the molecular level to what these two phylogenetically rather distant biomineralization systems have in common.