Partial amino acid sequences of six major subunits of bovine P-crystallin have been determined by automatic liquid-phase Edman degradation and the dansyl-Edman procedure, complemented by amino acid analyses of peptides. The results show that, including the previously established PBp sequence [H. P. Eur. J. Biochem. 121,, there exist at least seven primary gene products in bovine P-crystallin, which exhibit 40 % or more sequence homology. Two of the gene products are completely identical except for the presence in one of them of 17 additional residues at the N terminus, possibly caused by differential splicing of the same primary RNA transcript. The rate of evolutionary change of the fi chains (4 % sequence change per 100 x lo6 years) is about equally slow as that of a-crystallin, and the gene duplications giving rise to the different chains must have occurred very early in vertebrate evolution. The P chains can be divided into two groups, according to sequence homology and presence of deletions/insertions and C-terminal extension, on which basis a new, rational nomenclature for the /? subunits is introduced. The N-terminal extensions of all P chains are very different in length and sequence, even between homologous P chains in different species. Possible explanations for this finding are discussed.The evolutionarily highly conserved structural eye lens proteins, the crystallins, can be divided into four classes a, P, y and h-crystallin, of which P-crystallin is the most heterogeneous [I]. In bovine P-crystallin six or more chains are primary gene products, whereas some ten others arise by post-translational modification [2]. They can associate to oligomers, varying from dimers and trimers (filow) to octamers (fihigh) [2,3]. This diversity in P-crystallin subunits has also been found at the mRNA level in the chicken lens [4]. The primary structure of the predominant bovine P-crystallin chain, PBp, has been elucidated [5], while the protein structure of a major murine P-crystallin polypeptide (P 23) has been deduced from cDNA and genomic DNA analysis [6, 71. Several mRNAs of the rat lens crystallins have been cloned [8], and recently the nucleotide sequence of one of these has revealed the primary structure of the PBlchain (J. T. den Dunnen and J. G. G. Schoenmakers, unpublished data).Amino acid sequence determination of bovine y-crystallins [9, 101 and nucleotide sequence analyses of several y-crystallin cDNAs from rat [ll] and frog [12] have shown a considerable homology between P and y-crystallins. The three-dimensional structure of the bovine y-I1 chain has been determined [13] and a similar tertiary fold has been predicted for two / 3 chains, BBp [I41 and 823 [7]. The P and y-crystallins are built up of two domains, which show considerable sequence similarity, suggesting an intragenic duplication in the ancestral gene of these proteins. Each of the domains is again folded into two similar 'Greek key' motifs. A relationship between these structural motifs and the exons of the murine P23 gene has been suggeste...
Phylogenetic relationships among reptiles were examined using previously published and newly determined hemoglobin sequences. Trees reconstructed from these sequences using maximum-parsimony, neighbor-joining, and maximum-likelihood algorithms were compared with a phylogenetic tree of Amniota, which was assembled on the basis of published morphological data. All analyses differentiated alpha chains into alphaA and alphaD types, which are present in all reptiles except crocodiles, where only alphaA chains are expressed. The occurrence of the alphaD chain in squamates (lizards and snakes only in this study) appears to be a general characteristic of these species. Lizards and snakes also express two types of beta chains (betaI and betaII), while only one type of beta chain is present in birds and crocodiles. Reconstructed hemoglobin trees for both alpha and beta sequences did not yield the monophyletic Archosauria (i.e., crocodilians + birds) and Lepidosauria (i.e., Sphenodon + squamates) groups defined by the morphology tree. This discrepancy, as well as some other poorly resolved nodes, might be due to substantial heterogeneity in evolutionary rates among single hemoglobin lineages. Estimation of branch lengths based on uncorrected amino acid substitutions and on distances corrected for multiple substitutions (PAM distances) revealed that relative rates for squamate alphaA and alphaD chains and crocodilian beta chains are at least twice as high as those of the rest of the chains considered. In contrast to these rate inequalities between reptilian orders, little variation was found within squamates, which allowed determination of absolute evolutionary rates for this subset of hemoglobins. Rate estimates for hemoglobins of lizards and snakes yielded 1.7 (alphaA) and 3.3 (beta) million years/PAM when calibrated with published divergence time vs. PAM distance correlates for several speciation events within snakes and for the squamate left and right arrow sphenodontid split. This suggests that hemoglobin chains of squamate reptiles evolved approximately 3.5 (alphaA) or approximately 1.7 times (beta) faster than their mammalian equivalents. These data also were used to obtain a first estimate of some intrasquamate divergence times.
The origin of tetrapods has been debated for many years. In traditional systematics, the extinct lobe-finned bony fish (Rhipidistia) are regarded as the closest relatives of tetrapods. Among living fish, the coelacanth Latimeria chalumnae (Actinistia), which is the only recent representative of the Crossopterygii (Actinistia and Rhipidistia), the lungfish (Dipnoi) and ray-finned fish (Actinopterygii), have each been considered as sister-groups of the tetrapods. We have now determined the sequence of the alpha- and beta-globin chains of coelacanth haemoglobin and compared them with all known haemoglobins of bony and cartilaginous fish as well as those of tadpoles and adult amphibians. Haemoglobins of bony fish match more closely those of larval than adult amphibians. The beta chains of Latimeria match those of tadpoles more closely (54%) than do those of any other fish, whereas the alpha chains of Latimeria (45.4%), and especially of teleosts (49.2%), are closer to those of larval amphibians than are those of lungfish (39.8%). If only synapomorphous sequence matches (those at derived positions shared by one bony fish and tadpoles but not by any other bony fish) are considered, both Latimeria globin chains have distinctly more identities with phase of tadpoles than do those of any bony fish. Thus the primary structure of Latimeria haemoglobin indicates that the coelacanth is the closest living relative of tetrapods.
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