Background: Centromeres are essential for chromosome segregation, yet their DNA sequences evolve rapidly. In most animals and plants that have been studied, centromeres contain megabase-scale arrays of tandem repeats. Despite their importance, very little is known about the degree to which centromere tandem repeats share common properties between different species across different phyla. We used bioinformatic methods to identify high-copy tandem repeats from 282 species using publicly available genomic sequence and our own data. Results: Our methods are compatible with all current sequencing technologies. Long Pacific Biosciences sequence reads allowed us to find tandem repeat monomers up to 1,419 bp. We assumed that the most abundant tandem repeat is the centromere DNA, which was true for most species whose centromeres have been previously characterized, suggesting this is a general property of genomes. High-copy centromere tandem repeats were found in almost all animal and plant genomes, but repeat monomers were highly variable in sequence composition and length. Furthermore, phylogenetic analysis of sequence homology showed little evidence of sequence conservation beyond approximately 50 million years of divergence. We find that despite an overall lack of sequence conservation, centromere tandem repeats from diverse species showed similar modes of evolution. Conclusions: While centromere position in most eukaryotes is epigenetically determined, our results indicate that tandem repeats are highly prevalent at centromeres of both animal and plant genomes. This suggests a functional role for such repeats, perhaps in promoting concerted evolution of centromere DNA across chromosomes.

Hormonal control of transepithelial sodium (Na ⁺ ) transport utilizes phosphatidylinositide 3′-kinase (PI3K) and Raf–MAPK/ERK kinase (MEK)–ERK-dependent signaling pathways, which impact numerous cell functions. How signals transmitted by these pathways are sorted and appropriately transmitted to alter Na ⁺ transport without altering other physiologic processes is not well understood. Here, we report the identification of a signaling complex that selectively modulates the cell surface expression of the epithelial sodium channel (ENaC), an ion channel that is essential for fluid and electrolyte balance in mammals. Raf-1 and the ubiquitin ligase, Nedd4-2, are constitutively-expressed inhibitory components of this ENaC regulatory complex, which interact with, and decrease the expression of, cell surface ENaC. The activities of Nedd4-2 and Raf-1 are inhibited cooperatively by the PI3K-dependent kinase serum- and glucocorticoid-induced kinase 1 (SGK1), and the Raf-1-interacting protein glucocorticoid-induced leucine zipper (GILZ1), which are aldosterone-stimulated components of the complex. Together, SGK1 and GILZ1 synergistically stimulate ENaC cell surface expression. Interestingly, GILZ1 and SGK1 do not have synergistic, and in fact have opposite, effects on an unrelated activity, FKHRL1-driven gene transcription. Together, these data suggest that GILZ1 and SGK1 provide a physical and functional link between the PI3K- and Raf-1-dependent signaling modules and represent a unique mechanism for specifically controlling Na ⁺ transport without inappropriately activating other cell functions.

Glucocorticoid-induced leucine zipper protein (GILZ) is expressed in both epithelial and immune tissues and modulates a variety of cellular functions, including proliferation and epithelial sodium channel (ENaC) activity. A number of reports have described various GILZ activities, focusing on a single isoform with molecular mass of approximately 17 kDa, now termed GILZ1. In GILZ immunoblots using a newly developed antiserum, we detected multiple species in extracts from cultured kidney cells. Mass spectrometric analysis revealed that one of these represented a previously uncharacterized distinct isoform of GILZ, GILZ2. Rapid amplification of cDNA ends was used to clone cDNAs corresponding to four isoforms, which, in addition to GILZ1 and GILZ2, included new isoforms GILZ3 and GILZ4. Heterologous expression of these four GILZ isoforms in cultured cells revealed striking functional differences. Notably, GILZ1 was the only isoform that significantly stimulated ENaC-mediated Na+ current in a kidney collecting duct cell line, although GILZ2 and GILZ3 also stimulated ENaC surface expression in HEK 293 cells. GILZ1 and GILZ3, and to a lesser extent GILZ2, inhibited ERK phosphorylation. Interestingly, GILZ4, which had no effect on either ENaC or ERK, potently suppressed cellular proliferation, as did GILZ1, but not GILZ2 or GILZ3. Finally, rat and mouse tissues all expressed multiple GILZ species but varied in the relative abundance of each. These data suggest that multiple GILZ isoforms are expressed in most cells and tissues and that these play distinct roles in regulating key cellular functions, including proliferation and ion transport. Furthermore, GILZ inhibition of ERK appears to play an essential role in stimulation of cell surface ENaC but not in inhibition of proliferation.

Serum- and glucocorticoid-induced kinase 1 (SGK1) is a multifunctional protein kinase that markedly influences various cellular processes such as proliferation, apoptosis, glucose metabolism, and sodium (Na(+)) transport via the epithelial Na(+) channel, ENaC. SGK1 is a short-lived protein, which is predominantly targeted to the endoplasmic reticulum (ER) to undergo rapid proteasome-mediated degradation through the ER-associated degradation (ERAD) system. We show here that the aldosterone-induced chaperone, GILZ1 (glucocorticoid-induced leucine zipper protein-1) selectively decreases SGK1 localization to ER as well as its interaction with ER-associated E3 ubiquitin ligases, HRD1 and CHIP. GILZ1 inhibits SGK1 ubiquitinylation and subsequent proteasome-mediated degradation, thereby prolonging its half-life and increasing its steady-state expression. Furthermore, comparison of the effect of GILZ1 with that of proteasome inhibition (by MG-132) supports the idea that these effects of GILZ1 are secondary to physical interaction of GILZ1 with SGK1 and enhanced recruitment of SGK1 to targets within an "ENaC regulatory complex," thus making less SGK1 available to the ERAD machinery. Finally, effects of GILZ1 knockdown and overexpression strongly support the idea that these effects of GILZ1 are functionally important for ENaC regulation. These data provide new insight into how the manifold activities of SGK1 are selectively deployed and strengthened through modulation of its molecular interactions, subcellular localization, and stability.

In most species, the centromere is comprised of repetitive DNA sequences, which rapidly evolve. Paradoxically, centromeres fulfill an essential function during mitosis, as they are the chromosomal sites wherein, through the kinetochore, the mitotic spindles bind. It is now generally accepted that centromeres are transcribed, and that such transcription is associated with a broad range of functions. More than a decade of work on this topic has shown that centromeric transcripts are found across the eukaryotic tree and associate with heterochromatin formation, chromatin structure, kinetochore structure, centromeric protein loading, and inner centromere signaling. In this review, we discuss the conservation of small and long non-coding centromeric RNAs, their associations with various centromeric functions, and their potential roles in disease.