Abstract
The ERH gene encodes a highly conserved small nuclear protein with a unique amino acid sequence and three-dimensional structure but unknown function. The gene is present in animals, plants, and protists but to date has only been found in few fungi. Here we report that ERH homologs are also present in all four species from the genus Schizosaccharomyces, S. pombe, S. octosporus, S. cryophilus, and S. japonicus, which, however, are an exception in this respect among Ascomycota and Basidiomycota. The ERH protein sequence is moderately conserved within the genus (58% identity between S. pombe and S. japonicus), but the intron-rich genes have almost identical intron-exon organizations in all four species. In S. pombe, erh1+ is expressed at a roughly constant level during vegetative growth and adaptation to unfavorable conditions such as nutrient limitation and hyperosmotic stress caused by sorbitol. Erh1p localizes preferentially to the nucleus with the exception of the nucleolus, but is also present in the cytoplasm. Cells lacking erh1+ have an aberrant cell morphology and a comma-like shape when cultured to the stationary phase, and exhibit a delayed recovery from this phase followed by slower growth. Loss of erh1+ in an auxotrophic background results in enhanced arrest in the G1 phase following nutritional stress, and also leads to hypersensitivity to agents inducing hyperosmotic stress (sorbitol), inhibiting DNA replication (hydroxyurea), and destabilizing the plasma membrane (SDS); this hypersensitivity can be abolished by expression of S. pombe erh1+ and, to a lesser extent, S. japonicus erh1+ or human ERH. Erh1p fails to interact with the human Ciz1 and PDIP46/SKAR proteins, known molecular partners of human ERH. Our data suggest that in Schizosaccharomyces sp. erh1+ is non-essential for normal growth and Erh1p could play a role in response to adverse environmental conditions and in cell cycle regulation.
Highlights
The enhancer of rudimentary homolog (ERH) gene is present in genomes of most eukaryotic organisms including Animalia, Plantae and Protista with the noteworthy exception of Fungi in which it has not been described yet in such prominent model organisms as Saccharomyces cerevisiae, Schizosaccharomyces pombe or Aspergillus nidulans [1,2,3,4,5,6,7,8]
The genomes of two other members of the genus Schizosacccharomyces, S. japonicus and S. octosporus were sequenced as part of the Fungal Genome Initiative at the Broad Institute and their analysis revealed the presence of ERH-related sequences in both these species as loci SJAG_01867.4 and SOCG_03029.5, respectively [21]
In the light of the fact that these two species of the genus Schizosaccharomyces possess an ERH homolog we examined the genome of the third species of the genus, S. pombe, for the presence of an ERH homolog that could be overlooked during the initial automatic assembly of its ORFeome
Summary
The enhancer of rudimentary homolog (ERH) gene is present in genomes of most eukaryotic organisms including Animalia, Plantae and Protista with the noteworthy exception of Fungi in which it has not been described yet in such prominent model organisms as Saccharomyces cerevisiae, Schizosaccharomyces pombe or Aspergillus nidulans [1,2,3,4,5,6,7,8]. The novel gene was named enhancer of rudimentary and its protein product was proposed to be involved in the regulation of the metabolism of pyrimidines [1,10]. We have shown that in all human cells ERH could interact with two proteins, a DNA replication factor Ciz, and PDIP46/SKAR, a component of the exon junction complex involved in the regulation of cell growth, suggesting some functions of ERH in these fundamental processes [9,11,12,13]. ERH is a single domain protein which in accordance with its unique amino acid sequence, exhibits a novel a+b protein fold with the overall topology b1-310-b2-a1-a2-b3-b4a3 [5,14,15,16]
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