Nuclear mechanoprotection: From tissue atlases as blueprints to distinctive regulation of nuclear lamins

Two meters of DNA in each of our cells must be protected against many types of damage. Mechanoprotection is increasingly understood to be conferred by the nuclear lamina of intermediate filament proteins, but very different patterns of expression and regulation between different cells and tissues remain a challenge to comprehend and translate into applications. We begin with a tutorial style presentation of "tissue blueprints" of lamin expression including single-cell RNA sequencing in major public datasets. Lamin-A, C profiles appear strikingly similar to those for the mechanosensitive factors Vinculin, Yap1, and Piezo1, whereas datasets for lamin-B1 align with and predict regulation by the cell cycle transcription factor, FOXM1, and further predict poor survival across multiple cancers. Various experiments support the distinction between the lamin types and add mechanistic insight into the mechano-regulation of lamin-A, C by both matrix elasticity and externally imposed tissue strain. Both A- and B-type lamins, nonetheless, protect the nucleus from rupture and damage. Ultimately, for mechanically active tissue constructs and organoids as well as cell therapies, lamin levels require particular attention as they help minimize nuclear damage and defects in a cell cycle.

Self-Assembled Human Skin Equivalents Model Macrophage Activation of Cutaneous Fibrogenesis in Systemic Sclerosis

OBJECTIVE

The development of precision therapeutics for systemic sclerosis (SSc) has been hindered by the lack of models that accurately mimic the disease in vitro. This study was undertaken to design and test a self-assembled skin equivalent (saSE) system that recapitulates the cross-talk between macrophages and fibroblasts in cutaneous fibrosis.

METHODS

SSc-derived dermal fibroblasts (SScDFs) and normal dermal fibroblasts (NDFs) were cultured with CD14+ monocytes from SSc patients or healthy controls to allow de novo stroma formation. Monocyte donor-matched plasma was introduced at week 3 prior to seeding keratinocytes to produce saSE with a stratified epithelium. Tissue was characterized by immunohistochemical staining, atomic force microscopy, enzyme-linked immunosorbent assay, and quantitative reverse transcriptase-polymerase chain reaction.

RESULTS

Stroma synthesized de novo from NDFs and SScDFs supported a fully stratified epithelium to form saSE. A thicker and stiffer dermis was generated by saSE with SScDFs, and more interleukin-6 and transforming growth factor β (TGFβ) was secreted by saSE with SScDFs compared to saSE with NDFs, regardless of the inclusion of monocytes. Tissue with SSc monocytes and plasma had amplified dermal thickness and stiffness relative to control tissue. Viable CD163+ macrophages were found within the stroma of saSE 5 weeks after seeding. Additionally, SSc saSE contained greater numbers of CD163+ and CD206+ macrophages compared to control saSE. TGFβ blockade inhibited stromal stiffness to a greater extent in SSc saSE compared to control saSE.

CONCLUSION

These data suggest reciprocal activation between macrophages and fibroblasts that increases tissue thickness and stiffness, which is dependent in part on TGFβ activation. The saSE system may serve as a platform for preclinical therapeutic testing and for molecular characterization of SSc skin pathology through recapitulation of the interactions between macrophages and fibroblasts.

Filomicelles Deliver a Chemo-Differentiation Combination of Paclitaxel and Retinoic Acid That Durably Represses Carcinomas in Liver to Prolong Survival

Drug resistance and relapse is common in cancer treatments with chemotherapeutics, and while drug combinations with naturally occurring, differentiation-inducing retinoic acid (RA) provide remission-free cures for one type of liquid tumor, solid tumors present major problems for delivery. Here, inspired by filoviruses that can be microns in length, flexible filomicelles that self-assemble from an amphiphilic block copolymer (PEG-PCL) are shown to effectively deliver RA and paclitaxel (TAX) to several solid tumor models, particularly in the liver. These hydrophobic compounds synergistically load into the cores of the elongated micelles, and the coloaded micelles prove most effective at causing cell death, ploidy, and durable regression of tumors compared to free drugs or to separately loaded drugs. RA-TAX filomicelles also reduce mortality of human lung or liver derived cancers engrafted at liver, intraperitoneal, and subcutaneous sites in immunodeficient mice. In vitro studies show that the dual drug micelles effectively suppress proliferation while upregulating a generic differentiation marker. The results highlight the potency of dual-loaded filomicelles in killing cancer cells or else driving their differentiation away from growth.

Mechanobiology of bone marrow stem cells: from myosin-II forces to compliance of matrix and nucleus in cell forms and fates

Adult stem cells and progenitors are of great interest for their clinical application as well as their potential to reveal deep sensitivities to microenvironmental factors. The bone marrow is a niche for at least two types of stem cells, and the prototype is the hematopoietic stem cell/progenitors (HSC/Ps), which have saved many thousands of patients for several decades now. In bone marrow, HSC/Ps interact functionally with marrow stromal cells that are often referred to as mesenchymal stem cells (MSCs) or derivatives thereof. Myosin and matrix elasticity greatly affect MSC function, and these mechanobiological factors are now being explored with HSC/Ps both in vitro and in vivo. Also emerging is a role for the nucleus as a mechanically sensitive organelle that is semi-permeable to transcription factors which are modified for nuclear entry by cytoplasmic mechanobiological pathways. Since therapies envisioned with induced pluripotent stem cells and embryonic stem cells generally involve in vitro commitment to an adult stem cell or progenitor, a very deep understanding of stem cell mechanobiology is essential to progress with these multi-potent cells.

Gene signatures derived from a c-MET-driven liver cancer mouse model predict survival of patients with hepatocellular carcinoma

Biomarkers derived from gene expression profiling data may have a high false-positive rate and must be rigorously validated using independent clinical data sets, which are not always available. Although animal model systems could provide alternative data sets to formulate hypotheses and limit the number of signatures to be tested in clinical samples, the predictive power of such an approach is not yet proven. The present study aims to analyze the molecular signatures of liver cancer in a c-MET-transgenic mouse model and investigate its prognostic relevance to human hepatocellular carcinoma (HCC). Tissue samples were obtained from tumor (TU), adjacent non-tumor (AN) and distant normal (DN) liver in Tet-operator regulated (TRE) human c-MET transgenic mice (n = 21) as well as from a Chinese cohort of 272 HBV- and 9 HCV-associated HCC patients. Whole genome microarray expression profiling was conducted in Affymetrix gene expression chips, and prognostic significances of gene expression signatures were evaluated across the two species. Our data revealed parallels between mouse and human liver tumors, including down-regulation of metabolic pathways and up-regulation of cell cycle processes. The mouse tumors were most similar to a subset of patient samples characterized by activation of the Wnt pathway, but distinctive in the p53 pathway signals. Of potential clinical utility, we identified a set of genes that were down regulated in both mouse tumors and human HCC having significant predictive power on overall and disease-free survival, which were highly enriched for metabolic functions. In conclusions, this study provides evidence that a disease model can serve as a possible platform for generating hypotheses to be tested in human tissues and highlights an efficient method for generating biomarker signatures before extensive clinical trials have been initiated.

DLK1-DIO3 genomic imprinted microRNA cluster at 14q32.2 defines a stemlike subtype of hepatocellular carcinoma associated with poor survival

Hepatocellular carcinoma (HCC) is a heterogeneous and highly aggressive malignancy, for which there are no effective cures. Identification of a malignant stemlike subtype of HCC may offer patients with a dismal prognosis a potential targeted therapy using c-MET and Wnt pathway inhibitors. MicroRNAs (miRNAs) show promise as diagnostic and prognostic tools for cancer detection and stratification. Using a TRE-c-Met-driven transgenic HCC mouse model, we identified a cluster of 23 miRNAs that is encoded within the Dlk1-Gtl2 imprinted region on chromosome 12qF1 overexpressed in all of the isolated liver tumors. Interestingly, this region is conserved among mammalian species and maps to the human DLK1-DIO3 region on chromosome 14q32.2. We thus examined the expression of the DLK1-DIO3 miRNA cluster in a cohort of 97 hepatitis B virus-associated HCC patients and identified a subgroup (n = 18) of patients showing strong coordinate overexpression of miRNAs in this cluster but not in other cancer types (breast, lung, kidney, stomach, and colon) that were tested. Expression levels of imprinted gene transcripts from neighboring loci in this 14q32.2 region and from a subset of other imprinted sites were concomitantly elevated in human HCC. Interestingly, overexpression of the DLK1-DIO3 miRNA cluster was positively correlated with HCC stem cell markers (CD133, CD90, EpCAM, Nestin) and associated with a high level of serum α-fetoprotein, a conventional biomarker for liver cancer, and poor survival rate in HCC patients. In conclusion, our findings suggest that coordinate up-regulation of the DLK1-DIO3 miRNA cluster at 14q32.2 may define a novel molecular (stem cell-like) subtype of HCC associated with poor prognosis.

Characterization of microRNAs involved in embryonic stem cell states

Studies of embryonic stem cells (ESCs) reveal that these cell lines can be derived from differing stages of embryonic development. We analyzed common changes in the expression of microRNAs (miRNAs) and mRNAs in 9 different human ESC (hESC) lines during early commitment and further examined the expression of key ESCenriched miRNAs in earlier developmental states in several species. We show that several previously defined hESC-enriched miRNA groups (the miR-302, -17, and -515 families, and the miR-371-373 cluster) and several other hESC-enriched miRNAs are down-regulated rapidly in response to differentiation. We further found that mRNAs up-regulated upon differentiation are enriched in potential target sites for these hESC-enriched miRNAs. Interestingly, we also observed that the expression of ESC-enriched miRNAs bearing identical seed sequences changed dynamically while the cells transitioned through early embryonic states. In human and monkey ESCs, as well as human-induced pluripotent stem cells (iPSCs), the miR-371-373 cluster was consistently up-regulated, while the miR-302 family was mildly down-regulated when the cells were chemically treated to regress to an earlier developmental state. Similarly, miR-302b, but not mmu-miR-295, was expressed at higher levels in murine epiblast stem cells (mEpiSC) as compared with an earlier developmental state, mouse ESCs. These results raise the possibility that the relative expression of related miRNAs might serve as diagnostic indicators in defining the developmental state of embryonic cells and other stem cell lines, such as iPSCs. These data also raise the possibility that miRNAs bearing identical seed sequences could have specific functions during separable stages of early embryonic development.

Obesity and genetics regulate microRNAs in islets, liver, and adipose of diabetic mice

Type 2 diabetes results from severe insulin resistance coupled with a failure of b cells to compensate by secreting sufficient insulin. Multiple genetic loci are involved in the development of diabetes, although the effect of each gene on diabetes susceptibility is thought to be small. MicroRNAs (miRNAs) are noncoding 19-22-nucleotide RNA molecules that potentially regulate the expression of thousands of genes. To understand the relationship between miRNA regulation and obesity-induced diabetes, we quantitatively profiled approximately 220 miRNAs in pancreatic islets, adipose tissue, and liver from diabetes-resistant (B6) and diabetes-susceptible (BTBR) mice. More than half of the miRNAs profiled were expressed in all three tissues, with many miRNAs in each tissue showing significant changes in response to genetic obesity. Furthermore, several miRNAs in each tissue were differentially responsive to obesity in B6 versus BTBR mice, suggesting that they may be involved in the pathogenesis of diabetes. In liver there were approximately 40 miRNAs that were downregulated in response to obesity in B6 but not BTBR mice, indicating that genetic differences between the mouse strains play a critical role in miRNA regulation. In order to elucidate the genetic architecture of hepatic miRNA expression, we measured the expression of miRNAs in genetically obese F2 mice. Approximately 10% of the miRNAs measured showed significant linkage (miR-eQTLs), identifying loci that control miRNA abundance. Understanding the influence that obesity and genetics exert on the regulation of miRNA expression will reveal the role miRNAs play in the context of obesity-induced type 2 diabetes.

Obesity and genetics regulate microRNAs in islets, liver, and adipose of diabetic mice

Type 2 diabetes results from severe insulin resistance coupled with a failure of b cells to compensate by secreting sufficient insulin. Multiple genetic loci are involved in the development of diabetes, although the effect of each gene on diabetes susceptibility is thought to be small. MicroRNAs (miRNAs) are noncoding 19-22-nucleotide RNA molecules that potentially regulate the expression of thousands of genes. To understand the relationship between miRNA regulation and obesity-induced diabetes, we quantitatively profiled approximately 220 miRNAs in pancreatic islets, adipose tissue, and liver from diabetes-resistant (B6) and diabetes-susceptible (BTBR) mice. More than half of the miRNAs profiled were expressed in all three tissues, with many miRNAs in each tissue showing significant changes in response to genetic obesity. Furthermore, several miRNAs in each tissue were differentially responsive to obesity in B6 versus BTBR mice, suggesting that they may be involved in the pathogenesis of diabetes. In liver there were approximately 40 miRNAs that were downregulated in response to obesity in B6 but not BTBR mice, indicating that genetic differences between the mouse strains play a critical role in miRNA regulation. In order to elucidate the genetic architecture of hepatic miRNA expression, we measured the expression of miRNAs in genetically obese F2 mice. Approximately 10% of the miRNAs measured showed significant linkage (miR-eQTLs), identifying loci that control miRNA abundance. Understanding the influence that obesity and genetics exert on the regulation of miRNA expression will reveal the role miRNAs play in the context of obesity-induced type 2 diabetes.

Physical plasticity of the nucleus and its manipulation

The genome is virtually identical in all cells within an organism, with epigenetic changes contributing largely to the plasticity in gene expression during both development and aging. These changes include covalent modifications of chromatin components and altered chromatin organization as well as changes in other nuclear components, such as nuclear envelope lamins. Given that DNA in each chromosome is centimeters long and dozens of chromosomes are compacted into a microns-diameter nucleus through non-trivial interactions with the bounding envelope, the polymer physics of such a structure under stress can be complex but perhaps systematic. We summarize micromanipulation methods for measuring the physical plasticity of the nucleus, with recent studies documenting the extreme flexibility of human embryonic stem cells and the rigidification in model aging of progerin-type nuclei. Lamin-A/C is a common molecular factor, and methods are presented for its knockdown and measurement.

microRNAs regulate human embryonic stem cell division

microRNAs (miRNAs) regulate numerous physiological processes such as cell division and differentiation in many tissue types including stem cells. To probe the role that miRNAs play in regulating processes relevant to embryonic stem cell biology, we used RNA interference to silence DICER and DROSHA, the two main miRNA processing enzymes. Consistent with a role for miRNAs in maintaining normal stem cell division and renewal, we found that perturbation of miRNA pathway function in human embryonic stem cells (hESCs) attenuates cell proliferation. Normal cell growth can be partially restored by introduction of the mature miRNAs miR-195 and miR-372. These miRNAs regulate two tumor suppressor genes, respectively: WEE1, which encodes a negative G2/M kinase modulator of the CycB/CDK complex and CDKN1A, which encodes p21, a CycE/CDK cyclin dependent kinase inhibitor that regulates the G1/S transition. We show that in wild-type hESCs, WEE 1 levels control the rate of hESC division, whereas p21 levels must be maintained at a low level for hESC division to proceed. These data support a model for hESC cell cycle control in which miRNAs regulate negative cell cycle modulators at two phases of the cell cycle to ensure proper replenishment of the stem cell population.

Combinatorial microRNAs: working together to make a difference

MicroRNAs regulate gene networks and therefore are inherently complex. MicroRNAs themselves function in networks of other microRNAs, some of which are co-expressed from the same locus. To better understand the interplay among microRNAs that underlies their functions, we examined the potential of combinatorial effects of endogenously and exogenously co-expressed microRNAs. In this review, we first distill the similarities and differences between three microRNA families that function in cell division, miR-16, miR-34a and miR-106b, with emphasis on their exquisite phenotypic diversity. Given that the microRNAs affect cell cycle progression via distinct targets, we tested for phenotypic synergism among them. Furthermore, we investigate target regulation by individual and pooled microRNAs to gain insight into interactions among microRNAs co-expressed from the same chromosomal locus. The ability of microRNAs to modulate multiple genes within a molecular pathway engenders a novel way of thinking about targeting pathways: instead of a one-inhibitor-one-target model, multiple components in a pathway can be modulated by a microRNA resulting in a potent yet reversible inhibition of the pathway. To fully realize this potential, we need to understand how microRNAs function singly and in concert with each other.

Internal DNA pressure modifies stability of WT phage

dsDNA in bacteriophages is highly stressed and exerts internal pressures of many atmospheres (1 atm = 101.3 kPa) on the capsid walls. We investigate the correlation between packaged DNA length in lambda phage (78-100% of WT DNA) and capsid strength by using an atomic force microscope indentation technique. We show that phages with WT DNA are twice as strong as shorter genome mutants, which behave like empty capsids, regardless of high internal pressure. Our analytical model of DNA-filled capsid deformation shows that, because of DNA-hydrating water molecules, an osmotic pressure exists inside capsids that increases exponentially when the packaged DNA density is close to WT phage. This osmotic pressure raises the WT capsid strength and is approximately equal to the maximum breaking force of empty shells. This result suggests that the strength of the shells limits the maximal packaged genome length. Moreover, it implies an evolutionary optimization of WT phages allowing them to survive greater external mechanical stresses in nature.

Mitochondrial factors with dual roles in death and survival

At least in mammals, we have some understanding of how caspases facilitate mitochondria-mediated cell death, but the biochemical mechanisms by which other factors promote or inhibit programmed cell death are not understood. Moreover, most of these factors are only studied after treating cells with a death stimulus. A growing body of new evidence suggests that cell death regulators also have 'day jobs' in healthy cells. Even caspases, mitochondrial fission proteins and pro-death Bcl-2 family proteins appear to have normal cellular functions that promote cell survival. Here, we review some of the supporting evidence and stretch beyond the evidence to seek an understanding of the remaining questions.

Histone Modifications and the Chromatin Scaffold for Meiotic Chromosome Architecture

Chromosomes are capable of remarkable structural adaptability that enables their diverse functions. Histone modifications play pivotal roles in conferring structural diversity to chromosomes by influencing the compactness of chromatin. Several multi-protein complexes bind to chromatin and affect chromosome dynamics, including cohesin, condensin, the chromosome passenger complex, and the synaptonemal complex. The roles of these complexes in promoting chromosome functions include cohesion, condensation and synapsis. It is now crucial to define the relationship between the protein complexes that affect chromosome architecture and the underlying state of the chromatin. During meiosis chromosomes undergo striking morphological changes, including alignment of homologous chromosomes, double-strand break formation and repair, and establishment of meiosis-specific chromosome structures. These dynamic chromosome arrangements are accompanied by the recruitment and expulsion of multi-protein complexes from chromatin. Meiotic chromosome dynamics ensure proper chromosome segregation and production of healthy gametes. Meiosis thus affords an excellent opportunity to determine how histone modifications impact higher order chromosome dynamics by affecting localization and function of chromosome protein complexes. A meiotic mutation in the Drosophila histone kinase, NHK-1, uncovered a critical requirement for histone modifications in chromosome architecture, underscoring the power of this approach.

A histone code in meiosis: the histone kinase, NHK-1, is required for proper chromosomal architecture in Drosophila oocytes

To promote faithful propagation of the genetic material during sexual reproduction, meiotic chromosomes undergo specialized morphological changes that ensure accurate segregation of homologous chromosomes. The molecular mechanisms that establish the meiotic chromosomal structures are largely unknown. We describe a mutation in a recently identified Histone H2A kinase, nhk-1, in Drosophila that leads to female sterility due to defects in the formation of the meiotic chromosomal structures. The metaphase I arrest and the karyosome, a critical prophase I chromosomal structure, require nucleosomal histone kinase-1 (NHK-1) function. The defects are a result of failure to disassemble the synaptonemal complex and to load condensin onto the mutant chromosomes. Embryos laid by nhk-1-/- mutant females arrest with aberrant polar bodies and mitotic spindles, revealing that mitosis is affected as well. We analyzed the role of Histone H2A phosphorylation with respect to the histone code hypothesis and found that it is required for acetylation of Histone H3 and Histone H4 in meiosis. These studies reveal a critical role for histone modifications in chromosome dynamics in meiosis and mitosis.

The Drosophila MOS ortholog is not essential for meiosis

In metazoan oocytes, a metaphase arrest coordinates the completion of meiosis with fertilization. Vertebrate mos maintains the metaphase II arrest of mature oocytes and prevents DNA replication between the meiotic divisions. We identified a Drosophila homolog of mos and showed it to be the mos ortholog by two additional criteria. The dmos transcripts are present in Drosophila oocytes but not embryos, and injection of dmos into Xenopus embryos blocks mitosis and elevates active MAPK levels. In Drosophila, MAPK is activated in oocytes, consistent with a role in meiosis. We generated deletions of dmos and found that, as in vertebrates, dmos is responsible for the majority of MAPK activation. Unexpectedly, the oocytes that do mature complete meiosis normally and produce fertilized embryos that develop, although there is a reduction in female fertility and loss of some oocytes by apoptosis. Therefore, Drosophila contains a mos ortholog that activates a MAPK cascade during oogenesis and is nonessential for meiosis. This could be because there are redundant pathways regulating meiosis, because residual, low levels of active MAPK are sufficient, or because active MAPK is dispensable for meiosis in Drosophila. These results highlight the complexity of meiotic regulation that evolved to ensure accurate control over the reproductive process.

Fine structure analysis of the yeast centrin, Cdc31p, identifies residues specific for cell morphology and spindle pole body duplication

Centrin/Cdc31p is a Ca2+-binding protein related to calmodulin found in the MTOC of diverse organisms. In yeast, Cdc31p localizes to the SPB where it interacts with Kar1p and is required for SPB duplication. Recent findings suggest that centrin also functions elsewhere in the cell. To dissect the functions of Cdc31p, we generated cdc31 mutations chosen only for temperature sensitivity, but otherwise unbiased as to phenotype. Three phenotypes of the cdc31 mutants, temperature sensitivity, G2/M arrest, and cell lysis, were not well correlated, indicating that the mutations may differentially affect Cdc31p's interactions with other proteins. Alleles near the C-terminal region exhibited high G2/M arrest and genetic interactions with kar1-Delta17, suggesting that this region modulates an SPB-related function. Alleles causing high lysis and reduced Kic1p kinase activity mapped to the middle of the gene, suggesting disruption of a KIC1-like function and defects in activating Kic1p. A third region conferred temperature sensitivity without affecting cell lysis or G2/M arrest, suggesting that it defines a third function. Mutations in the C-terminal region were also defective for interaction with Kic1p. Mapping the alleles onto a predicted structure of Cdc31p, we have identified surfaces likely to be important for interacting with both Kar1p and Kic1p.

Functional interaction between the PKC1 pathway and CDC31 network of SPB duplication genes

The earliest known step in yeast spindle pole body (SPB) duplication requires Cdc31p and Kar1p, two physically interacting SPB components, and Dsk2p and Rad23p, a pair of ubiquitin-like proteins. Components of the PKC1 pathway were found to interact with these SPB duplication genes in two independent genetic screens. Initially, SLG1 and PKC1 were obtained as high-copy suppressors of dsk2Delta rad23Delta and a mutation in MPK1 was synthetically lethal with kar1-Delta17. Subsequently, we demonstrated extensive genetic interactions between the PKC1 pathway and the SPB duplication mutants that affect Cdc31p function. The genetic interactions are unlikely to be related to the cell-wall integrity function of the PKC1 pathway because the SPB mutants did not exhibit cell-wall defects. Overexpression of multiple PKC1 pathway components suppressed the G2/M arrest of the SPB duplication mutants and mutations in MPK1 exacerbated the cell cycle arrest of kar1-Delta17, suggesting a role for the PKC1 pathway in SPB duplication. We also found that mutations in SPC110, which encodes a major SPB component, showed genetic interactions with both CDC31 and the PKC1 pathway. In support of the model that the PKC1 pathway regulates SPB duplication, one of the phosphorylated forms of Spc110p was absent in pkc1 and mpk1Delta mutants.

A family of ubiquitin-like proteins binds the ATPase domain of Hsp70-like Stch

We have isolated two human ubiquitin-like (UbL) proteins that bind to a short peptide within the ATPase domain of the Hsp70-like Stch protein. Chap1 is a duplicated homologue of the yeast Dsk2 gene that is required for transit through the G2/M phase of the cell cycle and expression of the human full-length cDNA restored viability and suppressed the G2/M arrest phenotype of dsk2Delta rad23Delta Saccharomyces cerevisiae mutants. Chap2 is a homologue for Xenopus scythe which is an essential component of reaper-induced apoptosis in egg extracts. While the N-terminal UbL domains were not essential for Stch binding, Chap1/Dsk2 contains a Sti1-like repeat sequence that is required for binding to Stch and is also conserved in the Hsp70 binding proteins, Hip and p60/Sti1/Hop. These findings extend the association between Hsp70 members and genes encoding UbL sequences and suggest a broader role for the Hsp70-like ATPase family in regulating cell cycle and cell death events.

SLG1 plays a role during G1 in the decision to enter or exit the cell cycle

Saccharomyces cerevisiae cells decide to divide during G1. If nutrients are abundant, cells pass through START and coordinately undergo DNA replication, bud emergence, and spindle pole body duplication. Phenotypic analysis of the slg1delta mutant revealed that this mutation uncouples post-START events. At the nonpermissive temperature, slg1delta cells that have undergone bud emergence but not DNA replication or SPB duplication accumulate. Furthermore, while wild-type cells arrest in GO when starved, the slg1delta mutant fails to arrest at this point; instead, cells with small buds accumulate. The slg1delta mutation displayed genetic interactions with cdc34, which encodes a regulator of exit from G1. This is consistent with a role of SLG1 in G1 regulation. Epitope-tagged Slg1p cofractionated with the plasma membrane, suggesting that Slglp may function by integrating external cues and relaying them to the interior of the cell. We propose that SLG1 plays a regulatory role in bud emergence or stationary phase.

Yeast ubiquitin-like genes are involved in duplication of the microtubule organizing center

KAR1 is required for duplication of the Saccharomyces cerevisiae microtubule organizing center, the spindle pole body (SPB) (Rose, M.D., and G.R. Fink, 1987. Cell. 48:1047-1060). Suppressors of a kar1 allele defective for SPB duplication were isolated in two genes, CDC31 and DSK2 (Vallen, E.A., W.H., M. Winey, and M.D. Rose. 1994. Genetics. 137:407-422). To elucidate the role of DSK2 in SPB duplication, we cloned the gene and found it encodes a novel ubiquitin-like protein containing an NH2 terminus 36% identical to ubiquitin. The only other known yeast ubiquitin-like protein is encoded by the nucleotide excision repair gene RAD23 (Watkins, J.F.,P. Sung, L. Prakash, and S. Prakash. 1993. Mol. Cell. Bio. 13:7757-7765). Unlike ubiquitin, the NH2-terminal domain of Dsk2p is not cleaved from the protein, indicating that Dsk2p is not conjugated to other proteins. Although the DSK2-1 mutation alters a conserved residue in the Dsk2p ubiquitin-like domain, we detect no differences in Dsk2p or Cdc31p stability. Therefore, DSK2 does not act by interfering with ubiquitin-dependent protein degradation of these proteins. Although DSK2 is not essential, a strain deleted for both DSK2 and RAD23 is temperature sensitive for growth due to a block in SPB duplication. In addition, overexpression of DSK2 is toxic, and the DSK2-1 allele causes a block in SPB duplication. Therefore, DSK2 dosage is critical for SPB duplication. We determined that CDC31 gene function is downstream of DSK2 and KAR1. Dsk2p is a nuclear-enriched protein, and we propose that Dsk2p assists in Cdc31 assembly into the new SPB.