LINKER CHAIN L1 OF EARTHWORM HEMOGLOBIN - STRUCTURE OF GENE AND PROTEIN - HOMOLOGY WITH LOW-DENSITY-LIPOPROTEIN RECEPTOR

The extracellular hemoglobins (Hbs) of annelids and tube worms are gia nt multisubunit proteins of up to almost-equal-to 200 polypeptides and molecular masses to at least 3,900 kDa. They differ from all other Hb s in having both O2-binding chains and ''linker'' chains. The latter a re required for assembly and structural integrity of the protein and a re deficient in or lack heme. We have determined the nucleotide sequen ces of the cDNA and gene for linker chain L1 of the hemoglobin of Lumb ricus terrestris. The cDNA-derived amino acid sequence has 225 residue s and a calculated molecular mass of 25,847 Da. The chain is 21-28% id entical to linker chains of the related annelid Tylorrhynchus heteroch aetus and the deep-sea tube worm Lamellibrachia sp. A remarkable featu re of the linker chains is a conserved 38-39-residue segment that cont ains a repeating pattern of cysteinyl residues: (CYS-X6)3-Cys-X5-Cys-X 10-Cys. This pattern, not present in any globin sequence, corresponds exactly to the cysteine-rich repeats of the ligand binding domains of the low density lipoprotein (LDL) receptors of man and Xenopus laevis. Furthermore, the cysteine-rich segment of linker chain L1 has the seq uence Asp-Gly-Ser-Asp-Glu which is characteristic of LDL receptor repe ats. Similar cysteine-rich sequences also occur in two other mammalian proteins, complement C9 and renal glycoprotein GP330. The results sup port the conclusion that the cysteine-rich motif of the LDL receptor a nd annelid Hbs is a multipurpose protein-binding unit of ancient origi n which has been incorporated into diverse unrelated proteins, presuma bly by the process of exon shuffling.