Quaternary structure of rice nonsymbiotic hemoglobin

Plant nonsymbiotic hemoglobins are hexacoordinate heme proteins found in al l plants. Although expression is linked with hypoxic environmental conditio ns (Taylor, E. R., Nie, X. Z., Alexander, W. M., and Hill, R. D. (1994) Pla nt Mel. Biol. 24, 853-862), no discrete physio logical function has yet bee n attributed to this family of proteins. The crystal structure of a nonsymb iotic hemo globin from rice has recently been determined. The crystalline p rotein is homodimeric and hexacoordinate with two histidine side chains coo rdinating the heme iron atom. Despite the fact that the amino acids respons ible for the subunit interface are relatively conserved among the nonsymbio tic hemoglobins, previous work suggests that this group of proteins might d isplay variability in quaternary structure (Duff, S. M. G., Wittenberg, J. B., and Hill, R. D. (1997) J. Biol. Chem. 272, 16746-16752; Arredondo-Peter , R., Hargrove, M. S., Sarath, G., Moran, J. F., Lohrman, J., Olson, J. S., and Klucas, R. V. (1997) Plant Physiol. 115, 1259-1266). Analytical ultrac entrifugation and size exclusion high pressure liquid chromatography were u sed to investigate the quaternary structure of rice nonsymbiotic hemoglobin at various states of ligation and oxidation. Additionally, site-directed m utagenesis was used to test the role of several interface amino acids in di mer formation and ligand binding. Results were analyzed in light of possibl e physiological functions and indicate that the plant nonsymbiotic hemoglob ins are not oxygen transport proteins but more closely resemble known oxyge n sensors.