Isolation, characterization and N-terminal amino-acid sequence of rabbit transferrin

Presence of tear lipocalin and other major proteins in lacrimal fluid of rabbits

The lipocalins are a highly divergent, ubiquitous family of proteins that commonly function in binding lipophilic molecules. Although a specific tear lipocalin is a major component of lacrimal fluid and tears in many mammals, there has been no definitive identification of such a protein in rabbit tears. The goals of this project were to identify the major proteins in rabbit (Oryctolagus cuniculus) lacrimal fluid, so as to determine if they include a lipocalin and, if such a protein is present, to determine its source. Lacrimal fluid was collected from NZW sexually mature female rabbits, and culture medium from rabbit lacrimal gland epithelial (acinar) and interstitial cells was isolated. Proteins from these fluids were separated by SDS-PAGE electrophoresis and analyzed by sequencing the intact proteins and sequencing or mass analysis of fragments derived by trypsin digestion. Proteins of approximately 85 and 67 kDa were identified as rabbit transferrin and serum albumin, respectively, while components of 17 and 7 kDa had N-terminal sequences identical to those of lipophilin CL and AL, respectively. BLAST searches of the nr database with the N-terminal sequence of a protein of 18 kDa did not identify any homologues. However, when used to scan the PROSITE database, it was found to contain a lipocalin signature sequence. It is closely related to two lipocalins previously isolated from rabbit saliva and nasal mucus. Further studies with the N-terminal and internal sequences confirmed that the lacrimal protein is a lipocalin that is truncated at the N-terminus as compared with other tear lipocalins and is more similar to odorant binding proteins from rodents.

Comparison of transferrin sequences from different species

1. Amino acid sequences of transferrins from eight species, from human to tobacco hornworm, have been compared. Eighty-four amino acids (12%) are invariant, including three of the four ligands for the N-terminal Fe3+ ion. 2. The most highly conserved regions of both lobes of transferrin are the internal beta-sheets of domains 1 and 2, and helices 5 and 7 which abut the Fe3+ binding site. Two small patches of conserved surface residues, which may be involved in receptor binding, have also been identified. 3. Phylogenies have been deduced from pairwise alignment of the sequences of the N- and C-terminal lobes independently. The phylogenies are consistent with the evolutionary tree derived from the fossil record, and with the observation that the gene duplication which created the N- and C-terminal lobes of transferrin occurred before the divergence of the mammalian and insect lines. 4. The phylogenies predict that the lactotransferrin family diverged some 200 Myr ago, after the separation of the lines leading to mammals and birds. In contrast, the phylogenies predict that melanotransferrin diverged before the separation of the mammalian and avian lines. 5. Sequence comparisons also suggest that the stoichiometry of the transferrin receptor:transferrin complex is 2:1.

Iron carrier proteins facilitate engraftment of allogeneic bone marrow and enduring hemopoietic chimerism in the lethally irradiated host

Cell-free supernatants of rabbit bone marrow were fractionated, separated, and purified by Ultrogel and Superose chromatography. A single fraction promoted engraftment of allogeneic bone marrow and enduring hemopoietic chimerism across the H-2 barrier in lethally irradiated mice. This "bio-active" fraction, analyzed by reducing SDS-PAGE electrophoresis, and transblotted on PVDF membrane, and purified by reverse-phase HPLC and SDS-PAGE electrophoresis yielded a main prealbumin band that when examined for primary structure by Edman degradation, proved to be rabbit transferrin. This was also attested by highly specific precipitation of the prealbumin band with polyclonal antibodies to rabbit transferrin. Iron-saturated human transferrin, lactotransferrin, and egg transferrin (conalbumin) were assayed in irradiated C57BL/6 mice infused with bone marrow from histoincompatible BALB/c donors. Mice treated with iron-loaded transferrins survive and develop enduring allogeneic chimerism with no discernible signs of graft-versus-host disease. Iron carrier proteins thus provide an unique means of achieving successful engraftment of allogeneic bone marrow in immunologically hostile murine H-2 combinations.