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.
Highlights
Globins are small heme-proteins that have the ability to reversibly bind molecular oxygen (O2)
Recombinant expression of full-length D. rerio GbX in E. coli resulted in the formation of insoluble inclusion bodies
A second, minor band of,36 kDa suggests the formation of a stable dimer
Summary
Globins are small heme-proteins that have the ability to reversibly bind molecular oxygen (O2). For a long time only two globin types have been known in vertebrates: hemoglobin (Hb) and myoglobin (Mb). Hb and Mb are the best-studied proteins in biological, biochemical, biophysical and medical sciences. Hb, which resides in the cytoplasm of red blood cells and serves to transport O2 from the respiratory organs to the tissue, is certainly the best known globin type [1]. Vertebrate Hb is a tetramer composed of two a- and two b-chains. Mb is a monomer that is mainly located in the cytoplasm of the myocytes in heart or skeletal muscles. Mb stores O2, facilitates O2 diffusion to the mitochondria, and may be involved in the decomposition of NO [2]
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