Molecular Biology of the Ribonuclease A Gene Superfamily

在2005财年，我们继续探索核糖核酸酶RNase A家族的生物学和进化，这是一种罕见的酶家族，仅存在于脊椎动物物种中。虽然这些酶的化学性质已经被仔细地阐明，但这个高度分化的家族的生物学作用在很大程度上仍未被探索。我们的实验室一直处于这些生物学研究的前沿，建立在我们过去的历史基础上，包括人类EDN/RNase 2的分子克隆（PNAS 1989），人类ECP/RNase 3 (J Exp Med 1989)，灵长类动物中这一谱系的不寻常进化的阐明（自然遗传学1995）和啮齿动物（PNAS 2000），人类EDN的结构功能分析（PNAS 2002），人类RNase 6的鉴定和分子克隆（NAR 1996），人类RNase 8 (NAR 2002)，人类EDN和ECP作为抗病毒核糖核酸酶的特性（JID 1998; NAR 1998a, NAR 1998b）。

During FY2005, we continued our exploration of the biology and evolution of the RNase A family of ribonucleases, an unusual enzyme family that is restricted to vertebrate species. While the chemistry of these enzymes has been carefully elucidated, the biological role of this highly divergent family remains for the most part unexplored. Our laboratory remains in the forefront of these biological studies, building on our past history which includes such highlights as the molecular cloning of human EDN/RNase 2 (PNAS 1989), human ECP/RNase 3 (J Exp Med 1989), elucidation of the unusual evolution of this lineage in primates (Nature Genetics 1995) and in rodents (PNAS 2000), structure function analysis of human EDN (PNAS 2002), identification and molecular cloning of human RNase 6 (NAR 1996), human RNase 8 (NAR 2002), and characterization of human EDN and ECP as antiviral ribonucleases (JID 1998; NAR 1998a, NAR 1998b). The first of our published manuscripts during FY2005 details the evolutionary divergence of mouse RNase 6. M. musculus RNase 6 has a limited expression pattern compared to human RNase 6 and is an efficient ribonuclease, with a catalytic efficiency 17-fold higher than that of human protein. Evolutionary analysis reveals that RNase 6 was subject to unusual evolutionary forces (dN/dS = 1.2) in an ancestral rodent lineage before the separation of Mus and Rattus. However, more recent evolution of rodent RNase 6 has been relatively conserved, with an average dN/dS of 0.66. These data suggest that the ancestral rodent RNase 6 was subject to accelerated evolution, resulting in the conserved modern gene, which most likely plays an important role in mouse physiology. The second manuscript described the first case of exon splicing among the members of the RNase A superfamily. Conserved among humans, mice and rats, the RNase 4 and RNase 5/ang 1 locus includes two non-coding exons followed by two distinct exons encoding RNase 4 and RNase 5/ang 1. Transcription from this locus is controlled by differential splicing and tissue-specific expression from promoters located 5' to each of the non-coding exons. Promoter 1, 5' to exon I, is universally active, while Promoter 2, 5' to exon II, is active only in hepatic cells in promoter assays in vitro. Transcription from Promoter 2 is dependent on an intact HNF-1 consensus binding site which binds the transcription factor HNF-1alpha. In summary, RNase 4 and RNase 5/ang 1 are unique among the RNase A ribonuclease genes in that they maintain a complex gene locus that is conserved across species with transcription initiated from tissue-specific dual promoters followed by differential exon splicing. The third manuscript describes the intronless open reading frame encoding an RNase A ribonuclease from genomic DNA from the Iguana iguana IgH2 cell line. The iguana RNase is expressed primarily in pancreas, and represents the majority of the specific enzymatic activity in this tissue. The encoded sequence shares many features with its better-known mammalian counterparts including the crucial His12, Lys40 and His114 catalytic residues and efficient hydrolytic activity against yeast tRNA substrate (k(cat)/K(m)=6 x 10(4) M(-1) s(-1)), albeit at a reduced pH optimum (pH 6.0). Although the catalytic activity of the iguana RNase is not diminished by human placental RI, iguana RNase is not bactericidal nor is it cytotoxic even at micromolar concentrations. Phylogenetic analysis indicates moderate (46%) amino acid sequence similarity to a pancreatic RNase isolated from Chelydra serpentina (snapping turtle) although no specific relationship could be determined between these RNases and the pancreatic ribonucleases characterized among mammalian species. Further analysis of ribonucleases from non-mammalian vertebrate species is needed in order to define relationships and lineages within the larger RNase A gene superfamily. We also collaborated with Dr. M. Victoria Nogues in order to assess the involvement of some cationic and aromatic surface exposed residues of ECP in the inhibition of proliferation of mammalian cell lines. We have constructed ECP mutants for the selected residues and assessed their ability to prevent cell growth. Trp10 and Trp35 together with the adjacent stacking residue are critical for the damaging effect of ECP on mammalian cell lines. These residues are also crucial for the membrane disruption activity of ECP. Other exposed aromatic residues packed against arginines (Arg75-Phe76 and Arg121-Tyr122) and specific cationic amino acids (Arg101 and Arg104) of ECP play a secondary role in the cell growth inhibition. This may be related to the ability of the protein to bind carbohydrates such as those found on the surface of mammalian cells.