Lamin B methylation and assembly into the nuclear envelope

Post-Translational Modification of Lamins: Mechanisms and Functions

Lamins are the ancient type V intermediate filament proteins contributing to diverse biological functions, such as the maintenance of nuclear morphology, stabilization of chromatin architecture, regulation of cell cycle progression, regulation of spatial-temporal gene expressions, and transduction of mechano-signaling. Deregulation of lamins is associated with abnormal nuclear morphology and chromatin disorganization, leading to a variety of diseases such as laminopathy and premature aging, and might also play a role in cancer. Accumulating evidence indicates that lamins are functionally regulated by post-translational modifications (PTMs) including farnesylation, phosphorylation, acetylation, SUMOylation, methylation, ubiquitination, and O-GlcNAcylation that affect protein stabilization and the association with chromatin or associated proteins. The mechanisms by which these PTMs are modified and the relevant functionality become increasingly appreciated as understanding of these changes provides new insights into the molecular mechanisms underlying the laminopathies concerned and novel strategies for the management. In this review, we discussed a range of lamin PTMs and their roles in both physiological and pathological processes, as well as potential therapeutic strategies by targeting lamin PTMs.

The wide and growing range of lamin B-related diseases: from laminopathies to cancer

B-type lamins are fundamental components of the nuclear lamina, a complex structure that acts as a scaffold for organization and function of the nucleus. Lamin B1 and B2, the most represented isoforms, are encoded by LMNB1 and LMNB2 gene, respectively. All B-type lamins are synthesized as precursors and undergo sequential post-translational modifications to generate the mature protein. B-type lamins are involved in a wide range of nuclear functions, including DNA replication and repair, regulation of chromatin and nuclear stiffness. Moreover, lamins B1 and B2 regulate several cellular processes, such as tissue development, cell cycle, cellular proliferation, senescence, and DNA damage response. During embryogenesis, B-type lamins are essential for organogenesis, in particular for brain development. As expected from the numerous and pivotal functions of B-type lamins, mutations in their genes or fluctuations in their expression levels are critical for the onset of several diseases. Indeed, a growing range of human disorders have been linked to lamin B1 or B2, increasing the complexity of the group of diseases collectively known as laminopathies. This review highlights the recent findings on the biological role of B-type lamins under physiological or pathological conditions, with a particular emphasis on brain disorders and cancer.

Prominent role of histone lysine demethylases in cancer epigenetics and therapy

Protein methylation has an important role in the regulation of chromatin, gene expression and regulation. The protein methyl transferases are genetically altered in various human cancers. The enzymes that remove histone methylation have led to increased awareness of protein interactions as potential drug targets. Specifically, Lysine Specific Demethylases (LSD) removes methylated histone H3 lysine 4 (H3K4) and H3 lysine 9 (H3K9) through formaldehyde-generating oxidation. It has been reported that LSD1 and its downstream targets are involved in tumor-cell growth and metastasis. Functional studies of LSD1 indicate that it regulates activation and inhibition of gene transcription in the nucleus. Here we made a discussion about the summary of histone lysine demethylase and their functions in various human cancers.

Laminopathy-inducing mutations reduce nuclear import of expressed prelamin A

Lamins are structural components of the nuclear lamina and integral parts of the nucleoplasm. The tripartite domain structure partitions the molecule into an amino-terminal head, central rod and a carboxy-terminal tail domain. The tail domain contains a nuclear localization sequence and in most lamins an additional CaaX motif, which is necessary to post-translationally process prelamin to mature lamin. As players of nuclear and cellular integrity, lamins must possess unrestrained access to the nucleus. To study whether nuclear trafficking of lamins is compromised in laminopathies, we determined relative nuclear import activities between expressed prelamin A and selected laminopathy-inducing mutants thereof. Furthermore, the impact of inhibition of maturation on nuclear import of expressed prelamin A was examined. To perform quantitative transport measurements, import competent but lamina incorporation-deficient GFP- or DsRed-tagged prelamin A deletion mutants were used, which lacked the head and rod domain (ΔHR-prelamin A). Nuclear accumulation of ΔHR-prelamin A carrying the lipodystrophy and metabolic syndrome-inducing mutations R419C and L421P or progeria-causing deletions was significantly reduced, but that of the maturation-deficient mutant ΔHR-prelamin A SSIM was significantly increased. In the case of the full length prelamin A mutants R419C and L421P altered subcellular localization and reduced lamina incorporation were detected, with the prelamin A-binding protein Narf being redistributed into R419-containing aggregates. The results suggest that impaired nuclear transport of certain prelamin A mutants may represent a contributing factor in the pathogenesis of certain laminopathies.

Dynamic as well as stable protein interactions contribute to genome function and maintenance

The cell nucleus is responsible for the storage, expression, propagation, and maintenance of the genetic material it contains. Highly organized macromolecular complexes are required for these processes to occur faithfully in an extremely crowded nuclear environment. In addition to chromosome territories, the nucleus is characterized by the presence of nuclear substructures, such as the nuclear envelope, the nucleolus, and other nuclear bodies. Other smaller structural entities assemble on chromatin in response to required functions including RNA transcription, DNA replication, and DNA repair. Experiments in living cells over the last decade have revealed that many DNA binding proteins have very short residence times on chromatin. These observations have led to a model in which the assembly of nuclear macromolecular complexes is based on the transient binding of their components. While indeed most nuclear proteins are highly dynamic, we found after an extensive survey of the FRAP literature that an important subset of nuclear proteins shows either very slow turnover or complete immobility. These examples provide compelling evidence for the establishment of stable protein complexes in the nucleus over significant fractions of the cell cycle. Stable interactions in the nucleus may, therefore, contribute to the maintenance of genome integrity. Based on our compilation of FRAP data, we propose an extension of the existing model for nuclear organization which now incorporates stable interactions. Our new “induced stability” model suggests that self-organization, self-assembly, and assisted assembly contribute to nuclear architecture and function.

The posttranslational processing of prelamin A and disease

Human geneticists have shown that some progeroid syndromes are caused by mutations that interfere with the conversion of farnesyl-prelamin A to mature lamin A. For example, Hutchinson-Gilford progeria syndrome is caused by LMNA mutations that lead to the accumulation of a farnesylated version of prelamin A. In this review, we discuss the posttranslational modifications of prelamin A and their relevance to the pathogenesis and treatment of progeroid syndromes.

16 Inhibition of mammalian protein methyltransferases by 5'-methylthioadenosine (MTA): A mechanism of action of dietary same?

5'-deoxy-5'-methylthioadenosine (5'-methylthioadenosine, MTA) is a naturally occurring metabolite. As an experimental reagent, it has proved useful in providing investigators a window onto the role of protein methylation reactions in intact cells, although its mode of action is poorly understood in most cases. This chapter reevaluates its utility as a reagent. It appears now that MTA is at best a poor direct inhibitor of methyltransferases and that its effectiveness in intact cells may depend on its ability to inhibit S-adenosyl-l-homocysteine hydrolase. This chapter reviews recent evidence that points to an important role for MTA as an intermediary in the beneficial pharmaceutical action of orally ingested S-adenosyl-l-methionine (AdoMet, SAMe). These new results suggest that oral AdoMet may function not by enhancing the activity of cellular methyltransferases, as has been previously surmised, but by inhibiting their action. Such inhibition, particularly of protein methyltransferases involved in intracellular communication, may attenuate signal transduction pathways otherwise leading to inflammatory damage to tissues.

Nuclear Lamins: Laminopathies and Their Role in Premature Ageing

It has been demonstrated that nuclear lamins are important proteins in maintaining cellular as well as nuclear integrity, and in maintaining chromatin organization in the nucleus. Moreover, there is growing evidence that lamins play a prominent role in transcriptional control. The family of laminopathies is a fast-growing group of diseases caused by abnormalities in the structure or processing of the lamin A/C ( LMNA) gene. Mutations or incorrect processing cause more than a dozen different inherited diseases, ranging from striated muscular diseases, via fat- and peripheral nerve cell diseases, to progeria. This broad spectrum of diseases can only be explained if the responsible A-type lamin proteins perform multiple functions in normal cells. This review gives an overview of current knowledge on lamin structure and function and all known diseases associated with LMNA abnormalities. Based on the knowledge of the different functions of A-type lamins and associated proteins, explanations for the observed phenotypes are postulated. It is concluded that lamins seem to be key players in, among others, controlling the process of cellular ageing, since disturbance in lamin protein structure gives rise to several forms of premature ageing.

Prelamin A, Zmpste24, misshapen cell nuclei, and progeria--new evidence suggesting that protein farnesylation could be important for disease pathogenesis

Prelamin A undergoes multistep processing to yield lamin A, a structural protein of the nuclear lamina. Prelamin A terminates with a CAAX motif, which triggers farnesylation of a C-terminal cysteine (the C of the CAAX motif), endoproteolytic release of the last three amino acids (the AAX), and methylation of the newly exposed farnesylcysteine residue. In addition, prelamin A is cleaved a second time, releasing 15 more residues from the C terminus (including the farnesylcysteine methyl ester), generating mature lamin A. This second cleavage step is carried out by an endoplasmic reticulum membrane protease, ZMPSTE24. Interest in the posttranslational processing of prelamin A has increased with the recognition that certain progeroid syndromes can be caused by mutations that lead to an accumulation of farnesyl-prelamin A. Recently, we showed that a key cellular phenotype of these progeroid disorders, misshapen cell nuclei, can be ameliorated by inhibitors of protein farnesylation, suggesting a potential strategy for treating these diseases. In this article, we review the posttranslational processing of prelamin A, describe several mouse models for progeroid syndromes, explain the mutations underlying several human progeroid syndromes, and summarize recent data showing that misshapen nuclei can be ameliorated by treating cells with protein farnesyltransferase inhibitors.

Analysis of the localization and topology of nurim, a polytopic protein tightly associated with the inner nuclear membrane

Nurim is an inner nuclear membrane (INM) protein that was first isolated in a visual screen for nuclear envelope-localizing proteins. Nurim lacks an N-terminal domain characteristic of other INM proteins examined to date and may represent a class of proteins that localize to the INM by a distinct mechanism. To further characterize this protein, we constructed nurim-green fluorescent protein fusions and analyzed aspects of localization, biochemistry, and membrane topology. Results from immunoprobing and protease protection assays together with other analyses indicate that nurim (total length of 262 residues) is a six transmembrane-spanning protein and contains a hairpin turn in its C-terminal transmembrane domain, resulting in the N and C termini residing on the same side of the membrane. A loop region between the fourth and fifth transmembrane domains is exposed toward the nucleoplasm and contains a region accessible for site-specific endoproteinase cleavage. In biochemical fractionation, nurim remained extremely tightly bound to nuclear fractions and was released in significant quantities only in the presence of 4 m urea. Under conditions in which nuclear lamins were completely extracted, a significant population of nurim remained resistant to solubilization. This tight binding requires the C-terminal region of the protein. DNase treatment only marginally influenced its retention characteristics in nuclei. Results from consideration of sequence alignments and identification of specific topological features of nurim indicate that it may possess enzymic function. These results are discussed with reference to the retention mechanism and possible nuclear function of nurim.

Interleukin-1β induces posttranslational carboxymethylation and alterations in subnuclear distribution of lamin B in insulin-secreting RINm5F cells

We examined the effects of interleukin-1β (IL-1β) treatment on the distribution and degradation of lamin B in the nuclear fraction from insulin-secreting RINm5F cells. Western blot analysis indicated that IL-1β treatment caused significant alterations in the redistribution of lamin B, specifically between the Triton X-100-soluble (membrane) and -insoluble (matrix) fractions of the nucleus. IL-1β treatment also increased the lamin carboxymethyltransferase activity and the relative abundance of the carboxymethylated lamin in the nuclear fraction. A significant increase in the relative abundance of lamin B degradation products was also observed in the nuclear fraction from the IL-1β-treated cells. These findings are compatible with a measurable increase in the lamin-degrading caspase-6 activity in IL-1β-treated cells. Confocal microscopic observation of IL-1β-treated cells suggested a significant dissociation of lamin B from the nuclear lamina and its subsequent association with the DNA-rich elements within the nucleus. NG-monomethyl-l-arginine, a known inhibitor of inducible nitric oxide synthetase (iNOS), markedly inhibited IL-1β-induced iNOS gene expression, NO release, caspase-3 and caspase-6 activation, lamin B degradation, and loss of metabolic cell viability, indicating that the observed IL-1β-induced effects on nuclear lamin B involve the intermediacy of NO. Together, our data support the hypothesis that IL-1β treatment results in significant increase in the carboxymethylation of lamin B, which would place lamin B in a strategic location for its degradation mediated by caspases. This could possibly lead to dissolution of the nuclear envelope, culminating in the demise of the effete β-cell.

Partial cleavage of A-type lamins concurs with their total disintegration from the nuclear lamina during apoptosis

Although activated caspase 6 is capable of cleaving both A- and B-type lamins during apoptosis, the higher-order structure of the nuclear lamina may cause a differential breakdown of these two types of lamins. In order to obtain a better understanding of the dynamics and the consequences of the rapid, coordinated breakdown of the lamina complex, we applied the green fluorescent protein (GFP) technology in living cells, in which the fate of individual caspase cleavage fragments of A- and B-type lamins was examined. CHO-K1 cells were stably transfected with cDNA constructs encoding N-terminally GFP-labelled hybrids of lamin A, lamin Adelta10, lamin C or lamin B1. The course of the apoptotic process, induced by the kinase inhibitor staurosporine or by the proteasome inhibitor MG132, was monitored by digital imaging microscopy or confocal microscopy. Time-lapse recordings showed that parallel to DNA condensation N-terminally GFP-tagged A-type lamins became diffusely dispersed throughout the nucleoplasm and rapidly translocated to the cytoplasm. In contrast, the majority of GFP-lamin B1 signal remained localised at the nuclear periphery, even after extensive DNA condensation. Comparison of lamin B1-GFP signal with A-type lamin antibody staining in the same apoptotic cells confirmed the temporal differences between A- and B-type lamina dispersal. Immunoblotting revealed only a partial cleavage of A-type lamins and an almost complete cleavage of lamin B1 during apoptosis. In contrast to lamin B1 in normal cells, this cleaved lamin B1, which is apparently still associated with the nuclear membrane, can be completely extracted by methanol or ethanol. Fluorescence loss of intensity after photobleaching experiments showed that in apoptotic cells A-type lamin-GFP molecules diffuse almost freely in both nucleoplasm and cytoplasm, while the lamin B1-GFP fragments remain more stably associated with the nuclear membrane, which is confirmed by co-localisation immunofluorescence studies with a nucleoporin p62 antibody. Our results therefore clearly show a differential behaviour of A- and B-type lamins during apoptosis, suggesting not only distinct differences in the organisation of the lamina filaments, but also that caspase cleavage of only a small fraction of A-type lamins is needed for its complete disintegration.

5-Methylthiopentose: a new substituent on lipoarabinomannan in Mycobacterium tuberculosis

We have identified and characterised in several strains of Mycobacterium tuberculosis a new 5-methylthiopentose substituent on lipoarabinomannan (LAM). The 5-methylthiopentose was initially observed in heteronuclear (1)H-(13)C-NMR spectra of intact, (13)C-enriched LAM. Oligosaccharides carrying this substituent were released from (13)C-enriched LAM and from unlabelled LAM using an endo-arabinanase from Cellulomonas gellida. The presence of the methylthio group in these oligosaccharides was established using NMR, high-resolution Fourier-transform ion cyclotron resonance mass spectrometry and tandem mass spectrometry using a Q-TOF mass spectrometer. The 5-methylthiopentose is linked to a terminal mannose in the cap structures of these oligosaccharides as evidenced by tandem mass spectrometry and by NMR. We suggest interference with the signal transduction mechanisms of infected macrophages as a possible function for this newly discovered LAM substituent.

Evidence for the carboxyl methylation of nuclear lamin-B in the pancreatic beta cell

Lamins are intermediate filament proteins that constitute the main components of the lamina underlying the inner-nuclear membrane and serve to organize chromatin. Lamins (e.g., lamin-B) undergo posttranslational modifications (e.g., isoprenylation and methylation) at their C-terminal cysteine. Such modifications are thought to render optimal association of lamins with the nuclear envelop. Herein, we examined whether nuclear lamin-B undergoes carboxyl methylation in islet beta cells. A 65- to 70-kDa protein was carboxyl methylated in intact rat islets and clonal beta (HIT or INS) cells or in homogenates which could be immunoprecipitated using lamin-B antiserum. Incubation of purified HIT cell-nuclear fraction with [(3)H]S-adenosyl methionine yielded a single carboxyl methylated protein peak (ca. 65-70 kDa); this protein was immunologically identified as lamin-B. Several methylation inhibitors, including acetyl farnesyl cysteine, a competitive inhibitor of protein prenyl cysteine methylation, inhibited the carboxyl methylation of lamin-B, indicating that the carboxyl-methylated amino acid is cysteine. These findings, together with our recent observations demonstrating that inhibition of protein isoprenylation causes apoptotic death of the pancreatic beta cell, raise an interesting possibility that inhibition of C-terminal cysteine modifications of lamin-B might result in disruption of nuclear assembly, leading to further propagation of apoptotic signals, including DNA fragmentation and chromatin condensation.

Nuclear lamins: their structure, assembly, and interactions

Nuclear lamins are intermediate filament-type proteins that are the major building blocks of the nuclear lamina, a fibrous proteinaceous meshwork underlying the inner nuclear membrane. Lamins can also be localized in the nuclear interior, in a diffuse or spotted pattern. Nuclei assembled in vitro in the absence of lamins are fragile, indicating that lamins mechanically stabilize the cell nucleus. Available evidence also indicates a role for lamins in DNA replication, chromatin organization, spatial arrangement of nuclear pore complexes, nuclear growth, and anchorage of nuclear envelope proteins. In this review we summarize the current state of knowledge on the structure, assembly, and possible functional roles of nuclear lamins, emphasizing the information concerning the ability of nuclear lamins to self-assemble into distinct oligomers and polymers.

Induction of apoptosis in leukemia U937 cells by 5'-deoxy-5'-methylthioadenosine, a potent inhibitor of protein carboxylmethyltransferase

We found dramatic changes in leukemia U937 cells treated with 5'-deoxy-5'-methylthioadenosine (MTA), a potent inhibitor of protein carboxylmethyltransferase (protein methylase II). Initiation of cell death was observed by 1 day after MTA treatment, and it was induced in a dose- and time-dependent manner. However, cell viability measured by trypan blue exclusion was not consistent with the actual percentage of cell death. These results indirectly indicated that the type of cell death is apoptosis rather than necrosis. Nuclear fragmentation and DNA condensation of MTA-treated U937 cells were analyzed by both fluorescent and electron microscopy. MTA-treated cells first began to arrest in the M phase of the cell cycle, and they then exhibited a mitotic-like nuclear fragmentation process with partially membraneless chromatin. Furthermore, agarose gel electrophoresis of DNA extracted from cells treated with MTA showed DNA laddering with production of fragments of approximately 200 bp multiples. These studies indicated that cell death induced by MTA has the characteristics of apoptosis, although nuclear fragmentation is atypical. It seems likely that the process of apoptosis in U937 cells induced by MTA correlates with incomplete assembly of the nuclear envelope, since MTA itself could inhibit the carboxylmethylation of nuclear lamin B and delayed incorporation of lamin B into the nuclear envelope.

Inhibition by 5'-methylthioadenosine of cell growth and tyrosine kinase activity stimulated by fibroblast growth factor receptor in human gliomas

Stimulation of three human glioma cell lines with basic fibroblast growth factor (bFGF) led to the enhancement of cell growth and the rapid tyrosine phosphorylation of cellular proteins, including major substrates of 90 kD. A methyltransferase inhibitor, 5'-methylthioadenosine (MTA), inhibited dose dependently the bFGF-stimulated cell growth and protein tyrosine phosphorylation in glioma cells by blocking both receptor autophosphorylation and substrate phosphorylation, as shown by immunoblotting with antiphosphotyrosine antibodies and cross-linking bFGF to receptors. The antiproliferative activity of MTA correlated quantitatively with its potency as an inhibitor of bFGF-stimulated protein tyrosine kinase activity. The methyltransferase inhibitor MTA had no effect on either epidermal growth factor- or platelet-derived growth factor-stimulated protein tyrosine phosphorylation in glioma cells, but inhibited specifically bFGF-stimulated protein tyrosine kinase activity. The concentration of MTA required for inhibition of protein methylation correlated well with the concentration required for inhibition of bFGF-stimulated cell growth and protein tyrosine phosphorylation. Because MTA had no effect on numbers and dissociation constants of high- and low-affinity bFGF receptors, the inhibition of bFGF-stimulated bFGF receptor tyrosine kinase activity is not likely to be the result of a reduction in bFGF receptor and bFGF binding capacity. In fact, MTA delayed and reduced the internalization and nuclear translocation of bFGF, and the internalized bFGF was submitted to a limited proteolysis that converted it to lower molecular peptides whose presence remained for at least 22 hours. The effect of MTA on bFGF-stimulated tyrosine phosphorylation was immediate and readily reversible.

Prenylcysteine analogs to study function of carboxylmethylation in signal transduction

Carboxylmethylation of ras-related proteins is stimulated immediately on exposure of myeloid cells to inflammatory agonists. When the methylation reaction is inhibited with prenylcysteine analogs, G-protein-mediated signal transduction responses are disrupted, but responses to phorbol ester, calcium ionophore, and phospholipase C (PLC) remain intact. Furthermore, prenylcysteine analogs block GTP gamma S-induced aggregation of permeabilized platelets. Together, these results suggest that protein prenylcysteine methylation can play a role in signal transduction. A number of studies with AdoMet antagonists have suggested a role for methylation in cell-cycle regulation and stimulus-response coupling. Because the compounds generally inhibit all cellular methylation events, however, their effects have been difficult to interpret. On the other hand, prenylcysteine analogs have proved to be specific inhibitors of protein prenylcysteine methylation, as opposed to other types of methylation reactions. This enables the segregation of the role of methylation at C-terminal prenylcysteine residues from methylation at other sites, such as the carboxyl terminus of the catalytic subunit of PP2A. It should be emphasized, however, that prenylcysteine tails of proteins may interact with other target sites in addition to the methyltransferase enzyme(s), and prenylcysteine analogs may compete for these sites as well. One cannot assume that the inhibition of a response by the drugs necessarily implicates the involvement of a prenylcysteine methylation reaction. Studies with the analogs must be interpreted in conjunction with other results to ascertain the locus of their effects.

Protein methylation

Proteins can be enzymatically modified in several ways by the addition of methyl groups from S-adenosylmethionine. Reactions forming methyl esters on carboxyl groups are potentially reversible and can modulate the activity of the target protein; in the past year, advances have been made in understanding the physiological roles of four distinct systems that modify normal and abnormal carboxyl groups on proteins. On the other hand, methylation reactions occurring on nitrogen atoms in N-terminal and side-chain positions are generally irreversible. These reactions create new types of amino acid residues and can expand the repertoire of chemistry that a protein can perform.

Modification of eukaryotic signaling proteins by C-terminal methylation reactions

Eukaryotic polypeptides that are initially synthesized with the C-terminal sequence -Cys-Xaa-Xaa-Xaa, including a variety of signal-transducing proteins, such as small G-proteins, large G-proteins and cGMP phosphodiesterases, can be targeted for a series of sequential post-translational modifications. This processing pathway includes the isoprenylation of the cysteine residue with a farnesyl or geranylgeranyl moiety, followed by proteolysis of the three terminal residues and alpha-carboxyl methyl esterification of the cysteine residue. The potential reversibility of the last step suggests that it may be involved in modulating the function of these proteins. Firstly, methylation may play a role in the activation of cellular peptides or proteins. Secondly, this modification may aid in the membrane attachment of cytosolic precursor proteins. Thirdly, methylation may protect the polypeptide from C-terminal proteolytic degradation once the three terminal amino acid residues are removed. Finally, reversible methylation may directly regulate the function of its target proteins. Therapeutically, inhibitors of C-terminal isoprenylcysteine methylation or demethylation reactions may prove to be useful pharmacological tools as anti-cancer and anti-inflammatory agents.

Identification of an isoprenylated cysteine methyl ester hydrolase activity in bovine rod outer segment membranes

Proteins from eucaryotic cells which have a carboxyl-terminal CAAX motif are posttranslationally modified by isoprenylation. The pathway involves the linkage of an all-trans-farnesyl (C15) or an all-trans-geranylgeranyl (C20) moiety to the cysteine residue followed by proteolysis which generates the modified cysteine as the carboxyl-terminal residue. Carboxylmethylation of the modified cysteine residue completes the pathway. This latter methylation reaction is the only potentially reversible reaction in the pathway and thus of possible regulatory significance. A specific esterase is required to reverse the methylation. It is demonstrated here that simple isoprenylated cysteine derivatives, such as N-acetyl-S-farnesyl-L-cysteine methyl ester (L-AFCM) and N-acetyl-S-geranylgeranyl-L-cysteine methyl ester (L-AGGCM), are substrates for a rod outer segment (ROS) membrane esterase activity. The KM and Vmax values for L-AFCM and L-AGGCM are 186 microM and 2.2 nmol mg-1 min-1 and 435 microM and 4.8 nmol mg-1 min-1, respectively. The enzyme(s) is stereoselective rather than stereospecific because D-AFCM is enzymatically hydrolyzed with KM and Vmax values of 157 microM and 0.46 nmol mg-1 min-1, respectively. The enzyme(s) does not process N-acetyl-L-cysteine methyl ester, demonstrating that the isoprenyl moiety is required for substrate activity. Ebelactone B is a potent mechanism-based inactivator of the enzyme with a KI = 42 microM and a kinh = 3.7 x 10(-3) s-1. Importantly, L-AFCM, L-AGGCM, and ebelactone B all inhibit the demethylation of the endogenous ROS substrates, showing that the same enzymatic activity is involved in the processing of the synthetic and physiological substrates.

Analysis of stable protein methylation in cultured cells

It was demonstrated recently that substrates for protein N-methyltransferases (J. Najbauer and D. W. Aswad, 1990, J. Biol. Chem. 265, 12,717-12,721) and protein carboxyl methyltransferases (J. Najbauer, B. A. Johnson, and D. W. Aswad, 1991, Anal. Biochem. 197, 412-420) accumulate when rat PC12 cells are cultured in the presence of the methylation inhibitor, adenosine dialdehyde. In the present report, we have further characterized this phenomenon in PC12 cells and in two other, widely used cell types. Adenosine dialdehyde was found to increase the methyl-accepting capacity of proteins in human skin fibroblasts and mouse Sp2/0 myeloma cells. However, both the level of methyl incorporation in untreated cells and the amount of stimulation afforded by inhibitor treatment were substantially lower in these cells than in PC12 cells. All three cell lines accumulated methyl acceptor(s) at 17-21 kDa. The PC12 cells and the fibroblasts also exhibited stimulation of three apparently similar proteins in the 33- to 38-kDa region, where several arginine-methylated proteins involved in RNA processing would be expected. The optimal conditions for methylation of PC12 cell extracts with regard to pH, time of methylation, and S-[methyl-3H]adenosyl-L-methionine concentration were characterized. Increased methyl incorporation was detected after adenosine dialdehyde treatments as short as 2 h, and methylation of most substrates continued to increase as the time of treatment was extended to 72 h. The kinetics of accumulation varied from substrate to substrate. Fluorograms of two-dimensional gels of extracts from untreated PC12 cells incubated in the presence of S-[methyl-3H]adenosyl-L-methionine revealed patterns of methyl incorporation similar to those of treated cells, but longer exposure times were necessary (e.g., 35 days vs 7 days). These findings suggest that the inhibitor treatment works mainly by inhibiting the post- or cotranslational methylation of a "normal" array of cellular proteins.

Amplification and detection of substrates for protein carboxyl methyltransferases in PC12 cells

A strategy that facilitates the identification of substrates for protein carboxyl methyltransferases that form "stable" methyl esters, i.e., those that remain largely intact during conventional polyacrylamide gel electrophoresis is described. Rat PC12 cells were cultured in the presence of adenosine dialdehyde (a methylation inhibitor) to promote the accumulation of hypomethylated proteins. Nonidet P-40 cell extracts were then incubated in the presence of S-[methyl-3H]adenosyl-L-methionine to label methyl-accepting sites via endogenous methyltransferases. After labeled proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, gel slices were incubated in 4 N methanesulfonic acid or 6 N HCl to hydrolyze methyl esters. The resulting [3H]methanol was detected by trapping in liquid scintillation fluid. Seven carboxyl methylated proteins were observed with masses ranging from 18 to 96 kDa. Detection of five of these proteins required prior treatment of cells with adenosine dialdehyde, while methyl incorporation into one protein at 18 kDa was substantially enhanced by the treatment. The use of acidic conditions for methyl ester hydrolysis has an important advantage over assays that utilize alkaline hydrolysis conditions. In PC12 cells, and possibly other cell types where there are significant levels of arginine methylation, the methanol signal becomes obscured by high levels of volatile methylamines generated under the alkaline conditions. Carrying out diffusion assays under acidic conditions eliminates this interference. Adenosine dialdehyde, by virtue of increasing the methyl-accepting capacity of substrates for protein carboxyl methyltransferases, in combination with a more selective assay for carboxyl methylation, should prove useful in the isolation and characterization of new protein carboxyl methyltransferases and their substrates.

Characterization of a second highly conserved B-type lamin present in cells previously thought to contain only a single B-type lamin

Previous analyses of the nuclear lamina of mammalian cells have revealed three major protein components (lamins A, B and C) that have been identified by protein sequence homology as members of the intermediate filament (IF) protein family. It has been claimed that mammalian cells contain either all three lamins or lamin B alone. Using monoclonal antibodies specific for B-type lamins and cDNA cloning we identified a second major mammalian B-type lamin (murine lamin B2), thus showing that lamin composition in mammals is more complex than previously thought. Lamin B2 is coexpressed with lamin B1 (formerly termed lamin B) in all somatic cells and mammalian species that we analysed, including a variety of cells currently believed to contain only a single lamin. This suggests that two B-type lamins are necessary to form a functional lamina in mammalian somatic cells. By cDNA cloning we found that Xenopus laevis lamin LII is the amphibian homolog of mammalian lamin B2. Lamin expression during embryogenesis of amphibians and mammals shows striking similarities. The first lamins expressed in the early embryo are the two B-type lamins, while A-type lamins are only detected much later in development. These findings indicate that the genomic differentiation into two B-type lamins occurred early in vertebrate evolution and has been maintained in both their primary structure and pattern of expression.

Interphase nuclear envelope lamins form a discontinuous network that interacts with only a fraction of the chromatin in the nuclear periphery

Antibodies directed against nuclear envelope lamin proteins have been used in conjunction with three-dimensional light and electron microscope methodologies to determine the spatial organization of lamins in diploid interphase nuclei and to relate this organization to the positions of chromatin in the nuclear periphery. Using Drosophila early embryos, Drosophila Kc cells, and human HeLa cells, it is qualitatively and quantitatively observed that lamins are organized as a highly discontinuous, apparently fibrillar network that leaves large voids in the nuclear periphery containing little or no lamin. Using fluorescence microscopy to compare and quantitate the relationship between chromatin and the lamin network, it is found that although there is a strong tendency for the most peripheral chromatin to be positioned directly underneath a lamin fiber, only a small fraction of the chromatin in the nuclear periphery is sufficiently close to a lamin fiber to possibly be in direct contact.

In vitro disassembly of the nuclear lamina and M phase-specific phosphorylation of lamins by cdc2 kinase

The nuclear lamina is an intermediate filament-type network underlying the inner nuclear membrane. Phosphorylation of lamin proteins is believed to cause lamina disassembly during meiotic and mitotic M phase, but the M phase-specific lamin kinase has not been identified. Here we show that the cdc2 kinase, a major element implicated in controlling the eukaryotic cell cycle, phosphorylates chicken B-type lamins in vitro on sites that are specifically phosphorylated during M phase in vivo. Concomitantly, cdc2 kinase is capable of inducing lamina depolymerization upon incubation with isolated nuclei. One of the target sites of cdc2 kinase is identified as a motif (SPTR) conserved in the N-terminal domain of all lamin proteins. These results lead us to propose that mitotic disassembly of the nuclear lamina results from direct phosphorylation of lamins by cdc2 kinase.

Identification of cell cycle-regulated phosphorylation sites on nuclear lamin C

The mechanism by which MPF induces nuclear lamin disassembly and nuclear envelope breakdown during mitosis was studied in a frog egg extract in which the transition from interphase to mitosis can be induced by the addition of MPF. Bacterially expressed human nuclear lamin C, assembled in vitro into filaments, showed increased phosphorylation on specific sites in the extract in response to MPF. Phosphorylation was accompanied by disassembly of the lamin filaments. We determined the sequences of the sites phosphorylated both in the presence and absence of MPF. The sequence data suggest that multiple protein kinases act on the lamins, and S6 kinase II was identified as one potentially important lamin kinase.

Prenyl proteins in eukaryotic cells: a new type of membrane anchor

Recent studies have indicated that eukaryotic cells contain proteins that are post-translationally modified by long-chain, thioether-linked prenyl groups. These proteins include yeast mating factors, ras proteins and nuclear lamins. The modification occurs on a cysteine residue near the C terminus and appears to initiate a set of additional protein modification reactions that promote attachment of the proteins to specific membranes.

The function of protein carboxylmethyltransferase in eucaryotic cells

Protein carboxylmethyltransferase (PCM) is an enzyme whose function in eucaryotic cells remains controversial. Early studies suggested that protein carboxylmethylation subserved a regulatory, post-translational role in such diverse processes as secretion, neuronal receptor function, chemotaxis, and cellular differentiation. Later work strongly supported a totally unrelated role for this enzyme, i.e., the repair of spontaneously altered aspartate residues in cellular proteins. More recent evidence, however, suggests that a distinct, membrane-associated PCM catalyzes the methylation of alpha-carboxyl groups of C-terminal cysteines on discrete proteins. In view of these recent investigations, the data supporting a regulatory role for PCM are critically discussed and re-evaluated. There now appears to be compelling evidence that PCM(s) subserves both repair and regulatory functions in eucaryotic cells, catalyzing post-translational modifications of proteins involved in cell division, hormonal secretion, calmodulin-associated events and the interaction of guanyl nucleotide-linked proteins with the cell membrane.