Plant and Cell Physiology Advance Access originally published online on January 19, 2005
Plant and Cell Physiology 2005 46(1):87-98; doi:10.1093/pcp/pci002
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© 2005 Oxford University Press
The Rice Nucellin Gene Ortholog OsAsp1 Encodes an Active Aspartic Protease Without a Plant-specific Insert and is Strongly Expressed in Early Embryo
Plant Breeding, Genetics and Biochemistry Division, International Rice Research Institute, DAPO Box 7777, Metro Manila, Philippines
The barley nucellin gene was reported to be nucellus specific in its expression and was hypothesized to play a role in the programmed cell death of the nucellus as an aspartic protease. Here we provide direct evidence that the rice ortholog encodes an active aspartic protease, but we prefer the name aspartic protease1 (OsAsp1) to nucellin after a detailed analysis of its expression pattern in rice and barley. Northern blots, RT–PCR and RNA in situ hybridization showed that OsAsp1 is expressed most abundantly in zygotic embryos 1–2 d after fertilization. It is also expressed in pollen, nucellus, ovary wall, shoot and root meristem, coleoptiles of immature seeds, and somatic embryos. A parallel study in barley showed that the barley nucellin gene was expressed not only in the nucellus but also strongly in embryos. Recombinant protein proOsAsp1 expressed in the bacterium Escherichia coli refolded and autolysed at acidic pH 3.5 in vitro, and the mature peptide displayed protease activity. Nucellin has three close homologs in rice on chromosomes 11 and 12 and in Arabidopsis on chromosomes 1 and 4. They lack the plant-specific insert that distinguishes the typical plant aspartic protease from aspartic proteases of other organisms. They constitute a new class of aspartic protease that is present in both monocots and dicots but whose function remains to be explored further.
1 Corresponding author: E-mail, dbsbixz{at}nus.edu.sg; Fax, +65 68722013
2 Present address: Department of Biological Sciences, National University of Singapore, Singapore 117543
(Received March 8, 2004; Accepted October 8, 2004)
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