Abstract
The RIO family of atypical serine protein kinases has been first characterized only recently. It consists of enzymes that contain a unique domain with a characteristic kinase sequence motif and usually some additional domains. At least two RIO proteins, Rio1 and Rio2, are present in organisms varying from Archaea to humans, with a third Rio3 subfamily present only in multicellular eukaryotes. Yeast Rio1 and Rio2 proteins have been implicated in the processing of 20 S pre-rRNA and are necessary for survival of the cells. Crystal structures of Archaeoglobus fulgidus Rio1 and Rio2 have shown that whereas the overall fold of these enzymes resembles typical protein kinases, some of the structural domains, particularly those involved in peptide substrate binding, are not present. The mode of binding of nucleotides also differs from that found in typical protein kinases. Although it has been shown that both Rio1 and Rio2 have the enzymatic activity of kinases and are capable of autophosphorylation, the biological substrates of RIO proteins and their full biological role still remain to be discovered.
Highlights
The RIO family of proteins was first characterized only 8 years ago, based on the studies of a RIO1 gene, expressed constitutively at a low level in Saccharomyces cerevisiae [1]
Examination of their sequences reveals that the bacterial RIO kinases are most similar to Rio1 in the N-terminal half and more similar to Rio2 in the C-terminal half of the kinase domain (Fig. 1). It appears that the bacterial RIO kinases are related to both Rio1 and Rio2 enzymes and may represent the remnants of a common progenitor of the two subfamilies. This is interesting in view of the report that the Kdo lipid kinases bear significant homology to the bacterial RIO kinases [6], and the RIO kinases may represent the evolutionary link between bacterial lipid kinases and eukaryotic protein kinases (ePKs)
In yeast cells deprived of Rio1, cell cycle arrest occurs in G1 or mitosis, indicating that Rio1 activity is required for entry into S phase and exit from mitosis [3]
Summary
The founding member of the RIO family, Rio (sometimes called Rrp10p for ribosomal RNA processing number 10 [7]), is an essential gene in S. cerevisiae, required for proper cell cycle progression and chromosome maintenance [3]. In yeast cells deprived of Rio, cell cycle arrest occurs in G1 or mitosis, indicating that Rio activity is required for entry into S phase and exit from mitosis [3]. Both yeast Rio and Rio were identified as non-ribosomal factors necessary for late 18 S rRNA processing [5, 7]. The cellular localization of Rio and Rio in yeast was investigated using green fluorescent protein-labeled proteins. Both Rio and Rio shuttle between nucleus and cytoplasm. It was shown that Rio is a component of the 43 S pre-ribosomal subunit [10]
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