crustacyanin Type I CRTC and Type II CRTA subunits
Alpha crustacyanin bound with the carotenoid astaxanthisn (AXT) is the predominant cartenoprotein generating the slate-grey/blue color of the lobster carapace. Crustacyanin forms heterodimers (beta-crustacyanin) or complexes of 16 subunits (alpha-crustacyanin) assembled from beta-crustacyanin. Beta-crustacyanin is formed from one type I CRTC lipocalin subunit, and one type II CRTA lipocalin subunit (and two bound astaxanthin molecules). Homarus gammarus (European lobster) crustacyanin has of five distinct subunits evident on 6 M urea-PAGE gels: type I CRTC ( A1, C1, C2) and type II CRTA ( A2, A3). Homarus americanus crustacyanin consists of only two major subunits, namely type I CRTC (H1) and type II CRTA (H2), both of which behave like Ax subunits on a 6 M urea-PAGE gel. This family includes both type I CRTC subunit and type II CRTA subunits and belongs to the lipocalin/cytosolic fatty-acid binding protein family which have a large beta-barrel ligand-binding cavity. Lipocalins are mainly low molecular weight extracellular proteins that bind principally small hydrophobic ligands, and form covalent or non-covalent complexes with soluble macromolecules, as well as membrane bound-receptors. They participate in processes such as ligand transport, modulation of cell growth and metabolism, regulation of immune response, smell reception, tissue development and animal behavior. Cytosolic fatty-acid binding proteins, also bind hydrophobic ligands in a non-covalent, reversible manner, and have been implicated in intracellular uptake, transport and storage of hydrophobic ligands, regulation of lipid metabolism and sequestration of excess toxic fatty acids, as well as in signaling, gene expression, inflammation, cell growth and proliferation, and cancer development.
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