3-DIMENSIONAL RECONSTRUCTION OF NATIVE AND REASSEMBLED LUMBRICUS-TERRESTRIS EXTRACELLULAR HEMOGLOBIN - LOCALIZATION OF THE MONOMERIC GLOBINCHAINS

The similar to 3.5 MDa hexagonal bilayer (HBL) hemoglobin (Hb) of the earthworm Lumbricus terrestris is composed of monomers and disulfide-b onded trimers (T) of globin chains and of four types of heme-deficient linker chains (L). Cryoelectron microscopic images of native Hb and o f HBL reassembled from the constituent subunits depleted in monomer su bunit (HBL[T+L]) were subjected to three-dimensional reconstructions b y the random conical tilt series method. Native Hb has an architecture very similar to those of other annelid and vestimentiferan Hbs, consi sting of 12 hollow globular substructures (HGS). Each HGS is comprised of six dense masses, has a 3-fold symmetry, and is organized in two h exagonally symmetric layers, with the vertices of the upper layer rota ted 16 degrees clockwise relative to those of the lower layer. The lay ers are tethered to a central linker complex, consisting of two bracel ets of connections perpendicular to the 6-fold axis and a set of six v ertical connections linked to a flat hexagonal mass. HBL[T+L] shared a ll these features with the native Hb, except for a large hole around t he 3-fold symmetry axis in each HGS, indicating the probable location of the missing monomer subunit.