Isolation of intact larval haemoglobin from the brine shrimp Artemia salina. Prevention of degradation in vitro by proteases induced during larval development

Origin and evolution of the unique tetra-domain hemoglobin from the hydrothermal vent scale worm Branchipolynoe

Hemoglobin is the most common respiratory pigment in annelids. It can be intra or extracellular, and this latter type can form large multimeric complexes. The hydrothermal vent scale worms Branchipolynoe symmytilida and Branchipolynoe seepensis express an extracellular tetra-domain hemoglobin (Hb) that is unique in annelids. We sequenced the gene for the single-domain and tetra-domain globins in these two species. The single-domain gene codes for a mature protein of 137 amino acids, and the tetra-domain gene codes for a mature protein of 552 amino acids. The single-domain gene has a typical three exon/two intron structure, with introns located at their typical positions (B12.2 and G7.0). This structure is repeated four times in the tetra-domain gene, with no bridge introns or linker sequences between domains. The phylogenetic position of Branchipolynoe globins among known annelid globins revealed that, although extracellular, they cluster within the annelid intracellular globins clade, suggesting that the extracellular state of these Hbs is the result of convergent evolution. The tetra-domain structure likely resulted from two tandem duplications, domain 1 giving rise to domain 2 and after this the two-domain gene duplicated to produce domains 3 and 4. The high O(2) affinity of Branchipolynoe extracellular globins may be explained by the two key residues (B10Y and E7Q) in the heme pocket in each of the domains of the single and tetra-domain globins, which have been shown to be essential in the oxygen-avid Hb from the nematode Ascaris suum. This peculiar globin evolutionary path seems to be very different from other annelid extracellular globins and is most likely the product of evolutionary tinkering associated with the strong selective pressure to adapt to chronic hypoxia that characterizes hydrothermal vents.

Evolution of a polymeric globin in the brine shrimp Artemia

Several invertebrate species possess haemoglobins in which each polypeptide contains multiple haem-binding domains, possibly reflecting the fusion of multiple monomeric globin transcriptional units at the gene level. We have now analysed the transcript of such a polymeric globin gene from the brine shrimp Artemia, which expresses three polymeric haemoglobins, each of relative molecular mass 260,000 (Mr 260K). These are formed by the variable association of two different subunit types, alpha and beta (refs 2,3). Haemoglobins I and III are homodimers of alpha and beta subunit types, respectively, and haemoglobin II is a heterodimer (alpha beta). The individual globin chains are of similar size (Mr 130K), but the exact nature of the differences between the two subunit types is unclear. Analysis of complementary DNA clones encoding one of the subunits of the Artemia dimeric haemoglobin showed that the globin messenger RNA encodes nine myoglobin-like domains, connected by linking peptides. The residues in the linkers are characteristic of those found generally in such protein linkers, and include turn-promoting amino acids. Each domain also contains the conserved residues that are required for functional haem-binding, and from analysis of the sequences it was predicted that they all can adopt the classic myoglobin-like fold. Analysis of the derived amino-acid sequences indicated that the individual domains are duplicated monomers that fused to form the polymeric globin some 200 Myr ago. The fusion of multiple transcriptional units for the evolution of a polymeric globin gene may have been a general mechanism for the appearance of such polymeric haemoglobins in invertebrates.

The structure of Artemia sp. haemoglobins-I. Isolation and characterization of oxygen binding domains obtained by limited tryptic digestion

Limited tryptic digestion of the extracellular haemoglobins of the crustacea Artemia sp. result in series of discrete fragments which are multiples of 16,000 d. 2. These 16,000 d fragments, together with 50,000 d and 80,000 d fragments have similar amino acid composition and tryptic peptide maps as the intact globin chains. 3. The haem content of the 16,000 d fragments is the same as this of the intact pigment and they can bind oxygen in a non cooperative way. 4. These results strongly support that the 16,000 d fragments represent structural and functional units or domains from which the globin chains are built up.