Quantitative analysis of localization and nuclear aggregate formation induced by GFP-lamin A mutant proteins in living HeLa cells

PIGB maintains nuclear lamina organization in skeletal muscle of Drosophila

The nuclear lamina (NL) plays various roles and participates in nuclear integrity, chromatin organization, and transcriptional regulation. Lamin proteins, the main components of the NL, form a homogeneous meshwork structure under the nuclear envelope. Lamins are essential, but it is unknown whether their homogeneous distribution is important for nuclear function. Here, we found that PIGB, an enzyme involved in glycosylphosphatidylinositol (GPI) synthesis, is responsible for the homogeneous lamin meshwork in Drosophila. Loss of PIGB resulted in heterogeneous distributions of B-type lamin and lamin-binding proteins in larval muscles. These phenotypes were rescued by expression of PIGB lacking GPI synthesis activity. The PIGB mutant exhibited changes in lamina-associated domains that are large heterochromatic genomic regions in the NL, reduction of nuclear stiffness, and deformation of muscle fibers. These results suggest that PIGB maintains the homogeneous meshwork of the NL, which may be essential for chromatin distribution and nuclear mechanical properties.

Most myopathic lamin variants aggregate: a functional genomics approach for assessing variants of uncertain significance

Hundreds of LMNA variants have been associated with several distinct disease phenotypes. However, genotype-phenotype relationships remain largely undefined and the impact for most variants remains unknown. We performed a functional analysis for 178 variants across five structural domains using two different overexpression models. We found that lamin A aggregation is a major determinant for skeletal and cardiac laminopathies. An in vitro solubility assay shows that aggregation-prone variants in the immunoglobulin-like domain correlate with domain destabilization. Finally, we demonstrate that myopathic-associated LMNA variants show aggregation patterns in induced pluripotent stem cell derived-cardiomyocytes (iPSC-CMs) in contrast to non-myopathic LMNA variants. Our data-driven approach (1) reveals that striated muscle laminopathies are predominantly protein misfolding diseases, (2) demonstrates an iPSC-CM experimental platform for characterizing laminopathic variants in human cardiomyocytes, and (3) supports a functional assay to aid in assessing pathogenicity for myopathic variants of uncertain significance.

Drosophila p38 MAPK interacts with BAG-3/starvin to regulate age-dependent protein homeostasis

As organisms age, they often accumulate protein aggregates that are thought to be toxic, potentially leading to age‐related diseases. This accumulation of protein aggregates is partially attributed to a failure to maintain protein homeostasis. A variety of genetic factors have been linked to longevity, but how these factors also contribute to protein homeostasis is not completely understood. In order to understand the relationship between aging and protein aggregation, we tested how a gene that regulates lifespan and age‐dependent locomotor behaviors, p38 MAPK (p38Kb), influences protein homeostasis as an organism ages. We find that p38Kb regulates age‐dependent protein aggregation through an interaction with starvin, a regulator of muscle protein homeostasis. Furthermore, we have identified Lamin as an age‐dependent target of p38Kb and starvin.

Heterozygous lamin B1 and lamin B2 variants cause primary microcephaly and define a novel laminopathy

PURPOSE

Lamins are the major component of nuclear lamina, maintaining structural integrity of the nucleus. Lamin A/C variants are well established to cause a spectrum of disorders ranging from myopathies to progeria, termed laminopathies. Phenotypes resulting from variants in LMNB1 and LMNB2 have been much less clearly defined.

METHODS

We investigated exome and genome sequencing from the Deciphering Developmental Disorders Study and the 100,000 Genomes Project to identify novel microcephaly genes.

RESULTS

Starting from a cohort of patients with extreme microcephaly, 13 individuals with heterozygous variants in the two human B-type lamins were identified. Recurrent variants were established to be de novo in nine cases and shown to affect highly conserved residues within the lamin ɑ-helical rod domain, likely disrupting interactions required for higher-order assembly of lamin filaments.

CONCLUSION

We identify dominant pathogenic variants in LMNB1 and LMNB2 as a genetic cause of primary microcephaly, implicating a major structural component of the nuclear envelope in its etiology and defining a new form of laminopathy. The distinct nature of this lamin B-associated phenotype highlights the strikingly different developmental requirements for lamin paralogs and suggests a novel mechanism for primary microcephaly warranting future investigation.

Genotype-phenotype analysis of LMNA-related diseases predicts phenotype-selective alterations in lamin phosphorylation

Laminopathies are rare diseases associated with mutations in LMNA, which encodes nuclear lamin A/C. LMNA variants lead to diverse tissue-specific phenotypes including cardiomyopathy, lipodystrophy, myopathy, neuropathy, progeria, bone/skin disorders, and overlap syndromes. The mechanisms underlying these heterogeneous phenotypes remain poorly understood, although post-translational modifications, including phosphorylation, are postulated as regulators of lamin function. We catalogued all known lamin A/C human mutations and their associated phenotypes, and systematically examined the putative role of phosphorylation in laminopathies. In silico prediction of specific LMNA mutant-driven changes to lamin A phosphorylation and protein structure was performed using machine learning methods. Some of the predictions we generated were validated via assessment of ectopically expressed wild-type and mutant LMNA. Our findings indicate phenotype- and mutant-specific alterations in lamin phosphorylation, and that some changes in phosphorylation may occur independently of predicted changes in lamin protein structure. Therefore, therapeutic targeting of phosphorylation in the context of laminopathies will likely require mutant- and kinase-specific approaches.

A rapid solubility assay of protein domain misfolding for pathogenicity assessment of rare DNA sequence variants

PURPOSE

DNA sequencing technology has unmasked a vast number of uncharacterized single-nucleotide variants in disease-associated genes, and efficient methods are needed to determine pathogenicity and enable clinical care.

METHODS

We report an E. coli-based solubility assay for assessing the effects of variants on protein domain stability for three disease-associated proteins.

RESULTS

First, we examined variants in the Kv11.1 channel PAS domain (PASD) associated with inherited long QT syndrome type 2 and found that protein solubility correlated well with reported in vitro protein stabilities. A comprehensive solubility analysis of 56 Kv11.1 PASD variants revealed that disruption of membrane trafficking, the dominant loss-of-function disease mechanism, is largely determined by domain stability. We further validated this assay by using it to identify second-site suppressor PASD variants that improve domain stability and Kv11.1 protein trafficking. Finally, we applied this assay to several cancer-linked P53 tumor suppressor DNA-binding domain and myopathy-linked Lamin A/C Ig-like domain variants, which also correlated well with reported protein stabilities and functional analyses.

CONCLUSION

This simple solubility assay can aid in determining the likelihood of pathogenicity for sequence variants due to protein misfolding in structured domains of disease-associated genes as well as provide insights into the structural basis of disease.

The protein arginine methyltransferase PRMT6 inhibits HIV-1 Tat nucleolar retention

The human immunodeficiency virus (HIV)-1 transactivator protein Tat is known to play a key role in HIV infection, integrally related to its role in the host cell nucleus/nucleolus. Here we show for the first time that Tat localisation can be modulated by specific methylation, whereby overexpression of active but not catalytically inactive PRMT6 methyltransferase specifically leads to exclusion of Tat from the nucleolus. An R52/53A mutated Tat derivative does not show this redistribution, implying that R52/53, within Tat's nuclear/nucleolar localisation signal, are the targets of PRMT6 activity. Analysis using fluorescence recovery after photobleaching indicate that Tat nucleolar accumulation is largely through binding to nucleolar components, with methylation of Tat by PRMT6 preventing this. To our knowledge, this is the first report of specific protein methylation inhibiting nucleolar retention.

Pluripotent stem cells to model Hutchinson-Gilford progeria syndrome (HGPS): Current trends and future perspectives for drug discovery

Progeria, or Hutchinson-Gilford progeria syndrome (HGPS), is a rare, fatal genetic disease characterized by an appearance of accelerated aging in children. This syndrome is typically caused by mutations in codon 608 (p.G608G) of the LMNA, leading to the production of a mutated form of lamin A precursor called progerin. In HGPS, progerin accumulates in cells causing progressive molecular defects, including nuclear shape abnormalities, chromatin disorganization, damage to DNA and delays in cell proliferation. Here we report how, over the past five years, pluripotent stem cells have provided new insights into the study of HGPS and opened new original therapeutic perspectives to treat the disease.

Mitotic Stress Is an Integral Part of the Oncogene-Induced Senescence Program that Promotes Multinucleation and Cell Cycle Arrest

Oncogene-induced senescence (OIS) is a tumor suppression mechanism that blocks cell proliferation in response to oncogenic signaling. OIS is frequently accompanied by multinucleation; however, the origin of this is unknown. Here, we show that multinucleate OIS cells originate mostly from failed mitosis. Prior to senescence, mutant H-RasV12 activation in primary human fibroblasts compromised mitosis, concordant with abnormal expression of mitotic genes functionally linked to the observed mitotic spindle and chromatin defects. Simultaneously, H-RasV12 activation enhanced survival of cells with damaged mitoses, culminating in extended mitotic arrest and aberrant exit from mitosis via mitotic slippage. ERK-dependent transcriptional upregulation of Mcl1 was, at least in part, responsible for enhanced survival and slippage of cells with mitotic defects. Importantly, mitotic slippage and oncogene signaling cooperatively induced senescence and key senescence effectors p21 and p16. In summary, activated Ras coordinately triggers mitotic disruption and enhanced cell survival to promote formation of multinucleate senescent cells.

Mutations in LMNA modulate the lamin A--Nesprin-2 interaction and cause LINC complex alterations

Background

In eukaryotes the genetic material is enclosed by a continuous membrane system, the nuclear envelope (NE). Along the NE specific proteins assemble to form meshworks and mutations in these proteins have been described in a group of human diseases called laminopathies. Laminopathies include lipodystrophies, muscle and cardiac diseases as well as metabolic or progeroid syndromes. Most laminopathies are caused by mutations in the LMNAgene encoding lamins A/C. Together with Nesprins (Nuclear Envelope Spectrin Repeat Proteins) they are core components of the LINC complex (Linker of Nucleoskeleton and Cytoskeleton). The LINC complex connects the nucleoskeleton and the cytoskeleton and plays a role in the transfer of mechanically induced signals along the NE into the nucleus, and its components have been attributed functions in maintaining nuclear and cellular organization as well as signal transduction.

Results

Here we narrowed down the interaction sites between lamin A and Nesprin-2 to aa 403–425 in lamin A and aa 6146–6347 in Nesprin-2. Laminopathic mutations in and around the involved region of lamin A (R401C, G411D, G413C, V415I, R419C, L421P, R427G, Q432X) modulate the interaction with Nesprin-2 and this may contribute to the disease phenotype. The most notable mutation is the lamin A mutation Q432X that alters LINC complex protein assemblies and causes chromosomal and transcription factor rearrangements.

Conclusion

Mutations in Nesprin-2 and lamin A are characterised by complex genotype phenotype relations. Our data show that each mutation in LMNAanalysed here has a distinct impact on the interaction among both proteins that substantially explains how distinct mutations in widely expressed genes lead to the formation of phenotypically different diseases.

LMNA mutations in Polish patients with dilated cardiomyopathy: prevalence, clinical characteristics, and in vitro studies

Background

LMNA mutations are most frequently involved in the pathogenesis of dilated cardiomyopathy with conduction disease. The goal of this study was to identify LMNA mutations, estimate their frequency among Polish dilated cardiomyopathy patients and characterize their effect both in vivo and in vitro.

Methods

Between January, 2008 and June, 2012 two patient populations were screened for the presence of LMNA mutations by direct sequencing: 66 dilated cardiomyopathy patients including 27 heart transplant recipients and 39 dilated cardiomyopathy patients with heart failure referred for heart transplantation evaluation, and 44 consecutive dilated cardiomyopathy patients, referred for a family evaluation and mutation screening.

Results

We detected nine non-synonymous mutations including three novel mutations: p.Ser431*, p.Val256Gly and p.Gly400Argfs*11 deletion. There were 25 carriers altogether in nine families. The carriers were mostly characterized by dilated cardiomyopathy and heart failure with conduction system disease and/or complex ventricular arrhythmia, although five were asymptomatic. Among the LMNA mutation carriers, six underwent heart transplantation, fourteen ICD implantation and eight had pacemaker. In addition, we obtained ultrastructural images of cardiomyocytes from the patient carrying p.Thr510Tyrfs*42. Furthermore, because the novel p.Val256Gly mutation was found in a sporadic case, we verified its pathogenicity by expressing the mutation in a cellular model.

Conclusions

In conclusion, in the two referral centre populations, the screening revealed five mutations among 66 heart transplant recipients or patients referred for heart transplantation (7.6%) and four mutations among 44 consecutive dilated cardiomyopathy patients referred for familial evaluation (9.1%). Dilated cardiomyopathy patients with LMNA mutations have poor prognosis, however considerable clinical variability is present among family members.

Binding of p110 retinoblastoma protein inhibits nuclear import of simian virus SV40 large tumor antigen

Nuclear import of the simian virus 40 large tumor antigen (T-ag) is dependent on its nuclear localization signal (NLS) within amino acids 126-132 that is recognized by the importin alpha/beta1 heterodimer, as well as a protein kinase CK2 site at serine 112 upstream of the NLS, which enhances the interaction approximately 50-fold. Here we show for the first time that T-ag nuclear import is negatively regulated by N-terminal sequences (amino acids 102-110), which represent the binding site (BS) for the retinoblastoma (Rb) tumor suppressor protein (p110(Rb)). Quantitative confocal laser scanning microscopic analysis of the transport properties of T-ag constructs with or without Rb binding site mutations in living transfected cells or in a reconstituted nuclear transport system indicates that the presence of the RbBS significantly reduces nuclear accumulation of T-ag. A number of approaches, including the analysis of T-ag nuclear import in an isogenic cell pair with and without functional p110(Rb) implicate p110(Rb) binding as being responsible for the reduced nuclear accumulation, with the Ser(106) phosphorylation site within the RbBS appearing to enhance the inhibitory effect. Immunoprecipitation experiments confirmed association of T-ag and p110(Rb) and dependence thereof on negative charge at Ser(106). The involvement of p110(Rb) in modulating T-ag nuclear transport has implications for the regulation of nuclear import of other proteins from viruses of medical significance that interact with p110(Rb), and how this may relate to transformation.

Morphological analysis of 13 LMNA variants identified in a cohort of 324 unrelated patients with idiopathic or familial dilated cardiomyopathy

BACKGROUND

Mutations in the LMNA gene, encoding lamins A/C, represent a significant cause of dilated cardiomyopathy. We recently identified 18 protein-altering LMNA variants in a cohort of 324 unrelated patients with dilated cardiomyopathy. However, at least one family member with dilated cardiomyopathy in each of 6 pedigrees lacked the LMNA mutation (nonsegregation), whereas small sizes of 5 additional families precluded definitive determinations of segregation, raising questions regarding contributions by those variants to disease.

METHODS AND RESULTS

We have consequently expressed, in COS7 cells, GFP-prelamin A (GFPLaA) fusion constructs incorporating the 6 variants in pedigrees with nonsegregation (R101P, A318T, R388H, R399C, S437Hfsx1, and R654X), the 4 variants in pedigrees with unknown segregation (R89L, R166P [in 2 families], I210S, R471H), and 3 additional missense variants (R190Q, E203K, and L215P) that segregated with disease. Confocal immunofluorescence microscopy was used to characterize GFP-lamin A localization and nuclear morphology. Abnormal phenotypes were observed for 10 of 13 (77%) variants (R89L, R101P, R166P, R190Q, E203K, I210S, L215P, R388H, S437Hfsx1, and R654X), including 4 of 6 showing nonsegregation and 3 of 4 with uncertain segregation. All 7 variants affecting coil 1B and the lamin A-only mutation, R654X, exhibited membrane-bound GFP-lamin A aggregates and nuclear shape abnormalities. Unexpectedly, R388H largely restricted GFP-lamin A to the cytoplasm. Equally unexpected were unique streaked aggregates with S437Hfsx1 and giant aggregates with both S437Hfsx1 and R654X.

CONCLUSIONS

This work expands the recognized spectrum of lamin A localization abnormalities in dilated cardiomyopathy. It also provides evidence supporting pathogenicity of 10 of 13 tested LMNA variants, including some with uncertain or nonsegregation.

Nuclear protein import is reduced in cells expressing nuclear envelopathy-causing lamin A mutants

Lamins, which form the nuclear lamina, not only constitute an important determinant of nuclear architecture, but additionally play essential roles in many nuclear functions. Mutations in A-type lamins cause a wide range of human genetic disorders (laminopathies). The importance of lamin A (LaA) in the spatial arrangement of nuclear pore complexes (NPCs) prompted us to study the role of LaA mutants in nuclear protein transport. Two mutants, causing prenatal skin disease restrictive dermopathy (RD) and the premature aging disease Hutchinson Gilford progeria syndrome, were used for expression in HeLa cells to investigate their impact on the subcellular localization of NPC-associated proteins and nuclear protein import. Furthermore, dynamics of the LaA mutants within the nuclear lamina were studied. We observed affected localization of NPC-associated proteins, diminished lamina dynamics for both LaA mutants and reduced nuclear import of representative cargo molecules. Intriguingly, both LaA mutants displayed similar effects on nuclear morphology and functions, despite their differences in disease severity. Reduced nuclear protein import was also seen in RD fibroblasts and impaired lamina dynamics for the nucleoporin Nup153. Our data thus represent the first study of a direct link between LaA mutant expression and reduced nuclear protein import.

The flexible loop of the human cytomegalovirus DNA polymerase processivity factor ppUL44 is required for efficient DNA binding and replication in cells

Phosphoprotein ppUL44 of the human cytomegalovirus (HCMV) DNA polymerase plays an essential role in viral replication, conferring processivity to the DNA polymerase catalytic subunit pUL54 by tethering it to the DNA. Here, for the first time, we examine in living cells the function of the highly flexible loop of ppUL44 (UL44-FL; residues 162 to 174 [PHTRVKRNVKKAP(174)]), which has been proposed to be directly involved in ppUL44's interaction with DNA. In particular, we use a variety of approaches in transfected cells to characterize in detail the behavior of ppUL44Deltaloop, a mutant derivative in which three of the five basic residues within UL44-FL are replaced by nonbasic amino acids. Our results indicate that ppUL44Deltaloop is functional in dimerization and binding to pUL54 but strongly impaired in binding nuclear structures within the nucleus, as shown by its inability to form nuclear speckles, reduced nuclear accumulation, and increased intranuclear mobility compared to wild-type ppUL44. Moreover, analysis of cellular fractions after detergent and DNase treatment indicates that ppUL44Deltaloop is strongly reduced in DNA-binding ability, in similar fashion to ppUL44-L86A/L87A, a point mutant derivative impaired in dimerization. Finally, ppUL44Deltaloop fails to transcomplement HCMV oriLyt-dependent DNA replication in cells and also inhibits replication in the presence of wild-type ppUL44, possibly via formation of heterodimers defective for double-stranded DNA binding. UL44-FL thus emerges for the first time as an important determinant for HCMV replication in cells, with potential implications for the development of novel antiviral approaches by targeting HCMV replication.

Nuclear import of HSV-1 DNA polymerase processivity factor UL42 is mediated by a C-terminally located bipartite nuclear localization signal

The polymerase accessory protein of the human herpes simplex virus type 1 (HSV-1) DNA polymerase UL42 plays an essential role in viral replication, conferring processivity to the catalytic subunit UL30. We show here that UL42 is imported to the nucleus of living cells in a Ran- and energy-dependent fashion, through a process that requires a C-terminally located bipartite nuclear localization signal (UL42-NLSbip; PTTKRGRSGGEDARADALKKPK(413)). Moreover cytoplasmic mutant derivatives of UL42 lacking UL42-NLSbip are partially relocalized into the cell nucleus upon HSV-1 infection or coexpression with UL30, implying that the HSV-1 DNA polymerase holoenzyme can assemble in the cytoplasm before nuclear translocation occurs, thus explaining why the UL42 C-terminal domain is not strictly required for viral replication in cultured cells. However, mutation of both UL30 and UL42 NLS results in retention of the DNA polymerase holoenzyme in the cytoplasm, suggesting that simultaneous inhibition of both NLSs could represent a viable strategy to hinder HSV-1 replication. Intriguingly, UL42-NLSbip is composed of two stretches of basic amino acids matching the consensus for classical monopartite NLSs (NLSA, PTTKRGR(397); NLSB, KKPK(413)), neither of which are capable of targeting GFP to the nucleus on their own, consistent with the hypothesis that P and G residues in position +3 of monopartite NLSs are not compatible with nuclear transport in the absence of additional basic sequences located in close proximity. Our results showing that substitution of G or P of the NLS with an A residue partially confers NLS function will help to redefine the consensus for monopartite NLSs.

Specific contribution of lamin A and lamin C in the development of laminopathies

Mutations in the lamin A/C gene are involved in multiple human disorders for which the pathophysiological mechanisms are partially understood. Conflicting results prevail regarding the organization of lamin A and C mutants within the nuclear envelope (NE) and on the interactions of each lamin to its counterpart. We over-expressed various lamin A and C mutants both independently and together in COS7 cells. When expressed alone, lamin A with cardiac/muscular disorder mutations forms abnormal aggregates inside the NE and not inside the nucleoplasm. Conversely, the equivalent lamin C organizes as intranucleoplasmic aggregates that never connect to the NE as opposed to wild type lamin C. Interestingly, the lamin C molecules present within these aggregates exhibit an abnormal increased mobility. When co-expressed, the complex formed by lamin A/C aggregates in the NE. Lamin A and C mutants for lipodystrophy behave similarly to the wild type. These findings reveal that lamins A and C may be differentially affected depending on the mutation. This results in multiple possible physiological consequences which likely contribute in the phenotypic variability of laminopathies. The inability of lamin C mutants to join the nuclear rim in the absence of lamin A is a potential pathophysiological mechanism for laminopathies.

Mislocalization of human transcription factor MOK2 in the presence of pathogenic mutations of lamin A/C

BACKGROUND INFORMATION

hsMOK2 (human MOK2) is a DNA-binding transcriptional repressor. For example, it represses the IRBP (interphotoreceptor retinoid-binding protein) gene by competing with the CRX (cone-rod homeobox protein) transcriptional activator for DNA binding. Previous studies have shown an interaction between hsMOK2 and nuclear lamin A/C. This interaction could be important to explain hsMOK2 ability to repress transcription.

RESULTS

In the present study, we have tested whether missense pathogenic mutations of lamin A/C, which are located in the hsMOK2-binding domain, could affect the interaction with hsMOK2. We find that none of the tested mutations is able to disrupt hsMOK2 binding in vitro or in vivo. However, we observe an aberrant cellular localization of hsMOK2 into nuclear aggregates when pathogenic lamin A/C mutant proteins are expressed.

CONCLUSIONS

These results indicate that pathogenic mutations in lamin A/C lead to sequestration of hsMOK2 into nuclear aggregates, which may deregulate MOK2 target genes.

Histone-mediated transduction as an efficient means for gene delivery

Gene delivery into the nucleus of eukaryotic cells is inefficient, largely because of the significant barriers within the target cell of the plasma membrane and nuclear envelope. Recently, a group of basic proteins, including the HIV-1 Tat protein and the four core histones, have been shown to enter cells through a novel energy- and receptor-independent manner. Here, we show that engineered histone H2B proteins are able to mediate the efficient delivery of either green fluorescent protein or DNA into HeLa cells through the process of "Histone-Mediated Transduction" (HMT), with further enhancement achieved by utilizing a dimer of histones H2B and H2A. Subsequent nuclear delivery was accelerated approximately two-fold by the addition of an optimized nuclear localization signal to histone H2B, thereby increasing the affinity of interaction with components of the cellular nuclear import machinery, resulting in increased expression of a reporter gene. Further, we demonstrate that the domains responsible for this histone transduction are located in the N-terminal tail and globular regions of histone H2B. HMT represents a new, efficient, and technically non-demanding means to deliver DNA to the nucleus of intact cells, including embryonic stem cells, which has important applications in gene therapy and cancer therapeutics.

Pre-Lamin A processing is linked to heterochromatin organization

Pre-lamin A undergoes subsequent steps of post-translational modification at its C-terminus, including farnesylation, methylation, and cleavage by ZMPSTE24 metalloprotease. Here, we show that accumulation of different intermediates of pre-lamin A processing in nuclei, induced by expression of mutated pre-lamin A, differentially affected chromatin organization in human fibroblasts. Unprocessed (non-farnesylated) pre-lamin A accumulated in intranuclear foci, caused the redistribution of LAP2alpha and of the heterochromatin markers HP1alpha and trimethyl-K9-histone 3, and triggered heterochromatin localization in the nuclear interior. In contrast, the farnesylated and carboxymethylated lamin A precursor accumulated at the nuclear periphery and caused loss of heterochromatin markers and Lap2alpha in enlarged nuclei. Interestingly, pre-lamin A bound both HP1alpha and LAP2alpha in vivo, but the farnesylated form showed reduced affinity for HP1alpha. Our data show a link between pre-lamin A processing and heterochromatin remodeling and have major implications for understanding molecular mechanisms of human diseases linked to mutations in lamins.

Human cytomegalovirus DNA polymerase catalytic subunit pUL54 possesses independently acting nuclear localization and ppUL44 binding motifs

The catalytic subunit of human cytomegalovirus (HCMV) DNA polymerase pUL54 is a 1242-amino-acid protein, whose function, stimulated by the processivity factor, phosphoprotein UL44 (ppUL44), is essential for viral replication. The C-terminal residues (amino acids 1220-1242) of pUL54 have been reported to be sufficient for ppUL44 binding in vitro. Although believed to be important for functioning in the nuclei of infected cells, no data are available on either the interaction of pUL54 with ppUL44 in living mammalian cells or the mechanism of pUL54 nuclear transport and its relationship with that of ppUL44. The present study examines for the first time the nuclear import pathway of pUL54 and its interaction with ppUL44 using dual color, quantitative confocal laser scanning microscopy on live transfected cells and quantitative gel mobility shift assays. We showed that of two nuclear localization signals (NLSs) located at amino acids 1153-1159 (NLSA) and 1222-1227 (NLSB), NLSA is sufficient to confer nuclear localization on green fluorescent protein (GFP) by mediating interaction with importin alpha/beta. We also showed that pUL54 residues 1213-1242 are sufficient to confer ppUL44 binding abilities on GFP and that pUL54 and ppUL44 can be transported to the nucleus as a complex. Our work thus identified distinct sites within the HCMV DNA polymerase, which represent potential therapeutic targets and establishes the molecular basis of UL54 nuclear import.