Functional Diversification of Sea Lamprey Globins in Evolution and Development

Agnathans have a globin repertoire that markedly differs from that of jawed (gnathostome) vertebrates. The sea lamprey (Petromyzon marinus) harbors at least 18 hemoglobin, two myoglobin, two globin X, and one cytoglobin genes. However, agnathan hemoglobins and myoglobins are not orthologous to their cognates in jawed vertebrates. Thus, blood-based O transport and muscle-based O storage proteins emerged twice in vertebrates from a tissue-globin ancestor. Notably, the sea lamprey displays three switches in hemoglobin expression in its life cycle, analogous to hemoglobin switching in vertebrates. To study the functional changes associated with the evolution and ontogenesis of distinct globin types, we determined O binding equilibria, type of quaternary assembly, and nitrite reductase enzymatic activities of one adult (aHb5a) and one embryonic/larval hemoglobin (aHb6), myoglobin (aMb1) and cytoglobin (Cygb) of the sea lamprey. We found clear functional differentiation among globin types expressed at different developmental stages and in different tissues. Cygb and aMb1 have high O affinity and nitrite reductase activity, while the two hemoglobins display low O affinity and nitrite reductase activity. Cygb and aHb6 but not aHb5a show cooperative O binding, correlating with increased stability of dimers, as shown by gel filtration and molecular modeling. The high O-affinity and the lack of cooperativity confirm the identity of the sea lamprey aMb1 as O storage protein of the muscle. The dimeric structure and O-binding properties of sea lamprey and mammalian Cygb were very similar, suggesting a conservation of function since their divergence around 500million years ago.