title: Linker chains of the gigantic hemoglobin of the earthworm Lumbricus terrestris: primary structures of linkers L2, L3, and L4 and analysis of the connectivity of the disulfide bonds in linker L1.
abstract: The extracellular hemoglobin (Hb) of the earthworm, Lumbricus terrestris, has four major kinds of globin chains: a, b, c, and d, present in equimolar proportions, and additional non-heme, non-globin scaffolding chains called linkers that are required for the calcium-dependent assembly of the full-sized molecule. The amino acid sequences of all four of the globin chains and one of the linkers (L1) have previously been determined. The amino acid sequences via cDNA of each of the three remaining linkers, L2, L3, and L4, have been determined so that the sequences of all constituent polypeptides of the hemoglobin are now known. Each linker has a highly conserved cysteine-rich segment of approximately 40 residues that is homologous with the seven ligand-binding repeats of the human low-density lipoprotein receptor (LDLR). Analysis of linker L1 shows that the connectivity of the three disulfide bonds is exactly the same as in the LDLR ligand-binding repeats. The presence of a calcium-binding site comprising one glutamyl and three aspartyl residues in both the LDLR repeats and in the linkers supports the suggestion that calcium is required for the folding and disulfide connectivity of the linkers as in the LDLR repeats. Linker L2 is markedly heterogeneous and contains unusual glycine-rich sequences near the NH2-terminus and a polar zipper-like sequence with imperfect repeats of Asp-Asp-His at the carboxyl terminus. Similar Asp-Asp-His repeats have been found in a protein homologous to superoxide dismutase in the hemolymph of certain mussels. These repeats may function as metal-binding sites.
authors.affiliation: Institute of Cellular and Molecular Biology, Section of Neurobiology, School of Biological Sciences, University of Texas, Austin, Texas, USA.
authors.forename: Wen-Yen
authors.initials: WY
authors.lastname: Kao
authors.forename: Jun
authors.initials: J
authors.lastname: Qin
authors.forename: Kenzo
authors.initials: K
authors.lastname: Fushitani
authors.forename: Sandra S
authors.initials: SS
authors.lastname: Smith
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Gorr
authors.forename: Claire K
authors.initials: CK
authors.lastname: Riggs
authors.forename: James E
authors.initials: JE
authors.lastname: Knapp
authors.forename: Brian T
authors.initials: BT
authors.lastname: Chait
authors.forename: Austen F
authors.initials: AF
authors.lastname: Riggs
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001224:Aspartic Acid / Q000737:chemistry; D001483:Base Sequence; D001665:Binding Sites; D002118:Calcium / Q000737:chemistry / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D003545:Cysteine / Q000737:chemistry; D018076:DNA, Complementary / Q000378:metabolism; D004220:Disulfides / Q000737:chemistry; D004229:Dithiothreitol / Q000494:pharmacology; D018698:Glutamic Acid / Q000737:chemistry; D006418:Heme / Q000737:chemistry; D006454:Hemoglobins / Q000737:chemistry*; D006639:Histidine / Q000737:chemistry; D006801:Humans; D008024:Ligands; D013058:Mass Spectrometry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009835:Oligochaeta; D011485:Protein Binding; D011499:Protein Processing, Post-Translational; D017434:Protein Structure, Tertiary; D012333:RNA, Messenger / Q000378:metabolism; D011973:Receptors, LDL / Q000737:chemistry; D017386:Sequence Homology, Amino Acid; D012967:Sodium Dodecyl Sulfate / Q000737:chemistry; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D013482:Superoxide Dismutase / Q000737:chemistry
chemical_list: D018076:DNA, Complementary; D004220:Disulfides; D006454:Hemoglobins; D008024:Ligands; D012333:RNA, Messenger; D011973:Receptors, LDL; D001224:Aspartic Acid; D012967:Sodium Dodecyl Sulfate; D018698:Glutamic Acid; D006418:Heme; D006639:Histidine; D013482:Superoxide Dismutase; D003545:Cysteine; D002118:Calcium; D004229:Dithiothreitol