Minning D M, Goldberg D E
Howard Hughes Medical Institute, Departments of Molecular Microbiology and Medicine, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
J Biol Chem. 1998 Dec 4;273(49):32644-9. doi: 10.1074/jbc.273.49.32644.
The oxygen-avid, homooctameric hemoglobin of Ascaris (AH) has an unusual structure. Each polypeptide consists of two tandem globin folds followed by a highly charged COOH-terminal tail that contains four direct repeats of His-Lys-Glu-Glu (HKEE). Deletion analysis of the AH tail determined that at least two of the four HKEE repeats are required for efficient octamer formation. Surprisingly, the first four residues of the tail (Glu-His-His-Glu) alone were moderately effective in promoting multimerization. The hemoglobin from Pseudoterranova decipiens (PH) also consists of two globin domains followed by a shorter COOH-terminal extension containing only one HKEE repeat. Interchanging the tails of AH and PH revealed that the PH tail is moderately effective in promoting octamer formation. Dissociation analysis of wild-type and mutant AH and PH revealed that the intact octamers are stabilized by interactions between residues within the globin folds, not the tail. Mutational and biochemical studies revealed that one key interaction is contributed by isoleucine 15, which lies in the unusually long AB loop of AH. We propose that the AH tail plays no role in stabilization of the quaternary structure once formed but rather functions as an intramolecular chaperone, aiding assembly of the nascent AH octamer.
