Amphibian albumins as members of the albumin, alpha-fetoprotein, vitamin D-binding protein multigene family
TheXenopus laevis 68-kd and 74-kd albumin amino acid sequences are examined with respect to their relationship to the other known members of the albumin/α-fetoprotein/vitamin D-binding protein gene family. Each of the three members of this family presents a unique pattern of conserved regions indicating a differential selective pressure related to specific functional characteristics. Furthermore, an evolutionary tree of these genes was deduced from the divergence times calculated from direct nucleotide sequence comparisons of individual gene pairs. These calculations indicate that the vitamin D-binding protein/albumin separation occurred 560–600 million years (Myr) ago and the albumin/α-fetoprotein divergence 280 Myr ago. This observation leads to the hypothesis according to which the albumin/α-fetoprotein gene duplication occurred shortly after the amphibian/reptile separation. Consequently, and unlike mammals, amphibians and fishes should lack anα-fetoprotein in their serum at larval stages, which is consistent with a recent analysis of serum proteins inXenopus laevis larvae. This hypothesis now will have to be tested further in additional lower vertebrates.