A Membrane-Bound Vertebrate Globin

Abstract

The family of vertebrate globins includes hemoglobin, myoglobin, and other O2-binding proteins of yet unclear functions. Among these, globin X is restricted to fish and amphibians. Zebrafish (Danio rerio) globin X is expressed at low levels in neurons of the central nervous system and appears to be associated with the sensory system. The protein harbors a unique N-terminal extension with putative N-myristoylation and S-palmitoylation sites, suggesting membrane-association. Intracellular localization and transport of globin X was studied in 3T3 cells employing green fluorescence protein fusion constructs. Both myristoylation and palmitoylation sites are required for correct targeting and membrane localization of globin X. To the best of our knowledge, this is the first time that a vertebrate globin has been identified as component of the cell membrane. Globin X has a hexacoordinate binding scheme and displays cooperative O2 binding with a variable affinity (P 50∼1.3–12.5 torr), depending on buffer conditions. A respiratory function of globin X is unlikely, but analogous to some prokaryotic membrane-globins it may either protect the lipids in cell membrane from oxidation or may act as a redox-sensing or signaling protein.

Globin X is a six-coordinate globin that reduces nitrite to nitric oxide in fish red blood cells

“…2D). In contrast to the previous report (18), these data provide evidence that GbX is a newly discovered blood heme-globin.…”

“…4C). Previous reports described GbX as a membrane-bound globin (18); however, our results do not localize the protein in vivo at the plasma or nuclear membrane sites specifically. It should be noted that the cellular localization of GbX might be regulated by posttranslational modifications or alternative splicing of the N terminus segment that includes the putative myristoylation/palmitoylation motif.…”

“…S1) (17). During our analysis of zebrafish Globin X gene transcript (xgb) expression in different tissues, we were surprised to observe apparent expression in the RBCs, rather than restriction to the brain and retina as previously described (18). xgb mRNA can be identified by in situ hybridization at 48 h postfertilization (hpf) in the circulating cells within the main axial vessels (Fig.…”

“…It is tempting to speculate that GbX in RBCs may serve as a response mechanism to hypoxia, delivering oxygen at very low oxygen tensions when Hb is deoxygenated. The P 50 of zebrafish GbX ranges between 1.3 and 12.5 torr, depending on the pH and reducing conditions (18). This value is within the range of respiratory globins, but the low concentration of GbX (89.9 ± 26.3 nM in bass blood; Fig.…”

“…Zebrafish GbX was first characterized as a neuronal globin, found predominantly in the zebrafish eye and brain (18). However, the tissue distribution of GbX seems quite inconsistent compared with the two other species where GbX has been studied, namely goldfish (Carassius auratus) (2) and frog (Xenopus laevis) (28).…”