title: The rice nucellin gene ortholog OsAsp1 encodes an active aspartic protease without a plant-specific insert and is strongly expressed in early embryo.
abstract: 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.
authors.affiliation: Plant Breeding, Genetics and Biochemistry Division, International Rice Research Institute, DAPO Box 7777, Metro Manila, Philippines. dbsbixz@nus.edu.sg
authors.forename: Xuezhi
authors.initials: X
authors.lastname: Bi
authors.forename: Gurdev S
authors.initials: GS
authors.lastname: Khush
authors.forename: John
authors.initials: J
authors.lastname: Bennett
mesh_terms: D000595:Amino Acid Sequence; D016282:Aspartic Acid Endopeptidases / Q000235:genetics*; D001483:Base Sequence; D002874:Chromosome Mapping; D032461:Chromosomes, Plant / Q000235:genetics; D003001:Cloning, Molecular; D018744:DNA, Plant / Q000235:genetics; D018507:Gene Expression Regulation, Developmental; D015971:Gene Expression Regulation, Enzymologic; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant; D001467:Hordeum / Q000201:enzymology / Q000235:genetics; D008969:Molecular Sequence Data; D012275:Oryza / Q000196:embryology / Q000201:enzymology* / Q000235:genetics*; D010449:Peptide Mapping; D010802:Phylogeny; D010940:Plant Proteins / Q000235:genetics; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D018749:RNA, Plant / Q000235:genetics / Q000378:metabolism; D011994:Recombinant Proteins / Q000235:genetics; D017386:Sequence Homology, Amino Acid
chemical_list: D018744:DNA, Plant; D010940:Plant Proteins; D012333:RNA, Messenger; D018749:RNA, Plant; D011994:Recombinant Proteins; D016282:Aspartic Acid Endopeptidases; C109816:nucellin