title: The self-association of the giant hemoglobin from the earthworm, Lumbricus terrestris.
abstract: BACKGROUND
  The crystallographic structure of the gigantic hemoglobin (erythrocruorin) of the annelid worm, Lumbricus terrestris, provides a molar mass of 3.6MDa for the hexagonal bilayer structure. Prior to this determination, some light-scattering and ultracentrifugal measurements indicated higher masses: 4.1-4.4MDa. Values of 3.6MDa were attributed to dissociation or subunit loss. However, early electron microscopy of the giant hemoglobin from a related annelid, Eumenia crassa by Öster Levin, showed that the hexagonal bilayer molecules were present mostly as oligomers; few were monomeric.
  
  
  METHODS
  Measurements by light-scattering of solutions of Lumbricus hemoglobin resolved by size-exclusion chromatography have been used to determine the weight-average molar mass of self-associating proteins. The X-ray structure has been re-examined.
  
  
  RESULTS
  Our measurements show that both 3.6MDa monomers and self-association products are present as a mixture. Analysis of the X-ray structure indicates several different kinds of monomer-monomer interactions.
  
  
  CONCLUSIONS
  We propose that the measured masses of Lumbricus hemoglobin as high as 4.4MDa, result from oligomerization. These masses would result from the presence of an array of oligomers of various sizes together with monomers of 3.6MDa. Furthermore, several different kinds of monomer-monomer interactions are clearly evident in the X-ray structure as well as in solution.
  
  
  CONCLUSIONS
  The results demonstrate that self-association of monomers of the hemoglobin of Lumbricus terrestris explains the high molar masses of 4.1-4.4MDa previously observed.
authors.affiliation: Section of Neurobiology, University of Texas at Austin, 1 University Station CO920, Austin, TX 78712, USA. Electronic address: riggs@uts.cc.utexas.edu.
authors.forename: Austen F
authors.initials: AF
authors.lastname: Riggs
authors.affiliation: Section of Neurobiology, University of Texas at Austin, 1 University Station CO920, Austin, TX 78712, USA.
authors.forename: Claire K
authors.initials: CK
authors.lastname: Riggs
mesh_terms: D000818:Animals; D002850:Chromatography, Gel; D018360:Crystallography, X-Ray; D006454:Hemoglobins / Q000737:chemistry* / Q000302:isolation & purification; D008027:Light; D008958:Models, Molecular; D008970:Molecular Weight; D009835:Oligochaeta / Q000737:chemistry* / Q000378:metabolism; D055503:Protein Multimerization; D021122:Protein Subunits / Q000737:chemistry* / Q000302:isolation & purification; D012542:Scattering, Radiation
chemical_list: D006454:Hemoglobins; D021122:Protein Subunits; C064665:erythrocruorin
keywords: Dimers; Erythrocruorin; Light-scattering; Lumbricus hemoglobin; Molar mass; Self-association