Regulation of nuclear scaffold protease activity by calcium

Recent investigations have disclosed protease activity which is intimately associated with the nuclear scaffold. This protease activity shows a marked selectivity for lamins A/C, and from them may produce the 46-kDa NTPase thought to participate in nucleocytoplasmic RNA transport. Preliminary characterizations suggested that the protease activity might require Ca2+. In this report, we present evidence showing that nuclear scaffold protease activity is stringently regulated by Ca2+ within the physiologic range. Further, calmodulin antagonists such as calmidazolium and chlorpromazine produce marked inhibition of the protease activity, suggesting modulation by calmodulin or calmodulin-like factors.     

The 46-kDa nucleoside triphosphatase of rat liver nuclear scaffold represents the N-terminal portion of lamins A/C

The major nucleoside triphosphatase (NTPase) of rat liver nuclear scaffold (NS) or envelope, which is thought to participate in nucleocytoplasmic transport, has been identified via photoaffinity labeling as a 46-kDa polypeptide. This 46-kDa protein was purified by SDS-polyacrylamide gel electrophoresis and cleaved with trypsin. The resulting peptides were purified by HPLC and five were microsequenced. All five peptides appear to be derived from the N-terminal region of lamins A/C. Subsequent experiments with photolabeled NS showed that the 46-kDa polypeptide was selectively immunoprecipitated by antiserum specific to lamins A/C and by affinity-purified anti-lamin antibodies. Photolabeling of nuclei prepared in the presence of protease inhibitors showed predominant labeling of the 46-kDa polypeptide, suggesting that it is an integral nuclear constituent and not an artifact produced during NS preparation. Use of protease inhibitors throughout purification of NS increased the specificity of photolabeling of the 46-kDa band by significantly reducing photolabeling of smaller molecular weight components, which arise by proteolysis. Anti-lamin antibodies also produced a significant inhibition of NTPase activity in NS. These results suggest that the N-terminal portion of lamins A/C represents the 46-kDa NTPase, which, according to previous reports, may participate in RNA transport.     

The Mr 46,000 nuclear scaffold ATP-binding protein: identification of the putative nucleoside triphosphatase by proteolysis and monoclonal antibodies directed against lamins A/C

Previous work suggested that the major Mr 46,000 ATP-binding protein [a putative nucleoside triphosphatase (NTPase)] found in rat liver nuclear scaffold (NS) may be proteolytically derived from lamins A/C. To definitively establish this identification, we undertook a series of photolabeling, proteolysis, and immunoprecipitation experiments. Mice were immunized with human lamin C expressed in bacteria, and monoclonal antibody-producing hybridomas were obtained. The purified monoclonal antibodies all recognized lamins A and C on immunoblots of NS, as well as Mr 46,000 or 34,000 proteolytic fragments as minor components. The Mr 46,000 photolabeled band was the only major NS component photolabeled with low concentrations of azido-ATP, and it was immunoprecipitated with anti-lamin monoclonal antibodies. To preclude the possibility that the photolabeled Mr 46,000 protein represented a minor component which comigrated with the Mr 46,000 lamin fragment and which specifically associated with lamins A/C during immunoprecipitation, a series of proteolytic digestions were undertaken. Digestion of the photolabeled Mr 46,000 peptide with chymotrypsin and staphylococcal protease V8 produced a limited number of photolabeled fragments, all of which comigrated with major stainable fragments produced from the Mr 46,000 lamin fragment. Cyanogen bromide cleavage of the photolabeled Mr 46,000 polypeptide, followed by polyacrylamide gel electrophoresis or high performance liquid chromatography/amino acid analyses, defined the COOH-terminal cleavage site as the Y residue at amino acid 376 and localized the photolabeled site to the COOH-terminal region (amino acids 372-376). In support of this proposed proteolytic cleavage site, specific assays with tyrosine-containing thiobenzyl ester substrate documented the presence of NS protease activity which cleaves at tyrosine residues; this activity shows a Km of 0.2 mM and a Kcat of approximately 250/s. Parallel experiments with mildly proteolyzed cloned lamin C preparations showed selective photolabeling of an Mr 34,000 fragment, which corresponds to a proteolytic breakdown product of the Mr 46,000 NS polypeptide; this Mr 34,000 photolabeled fragment was also immunoprecipitated with anti-lamin monoclonal antibodies and contained the same photolabeled site as the Mr 46,000 peptide. Cloned lamin C preparations were inactive in NTPase assays but did exhibit substantial ATP binding with an apparent KD = 4 x 10(-5) M ATP. These results indicate that the major Mr 46,000 photoaffinity-labeled protein in NS, which represents the putative NTPase thought to participate in nucleocytoplasmic transport, is derived from lamin A or lamin C by NS proteolytic activity which exposes a cryptic ATP-binding site near the highly conserved end of coil-2.(ABSTRACT TRUNCATED AT 400 WORDS)     

Role of nuclear lamins in nuclear segmentation of human neutrophils

Nuclear breakdown leading to the formation of apoptotic bodies has been postulated to involve degradation of nuclear structural proteins, such as lamins A/C and B. Although nuclear segmentation occurs during the maturation of polymorphonuclear leukocytes (neutrophils), its mechanism is not known. We found that human neutrophils have lamin B but lack lamins A/C while mononuclear cells possess all three types of lamin as assessed by immunoblotting. Differentiation of human promyelocytic HL-60 cells into neutrophil-like cells was also accompanied by the down-regulation of lamins A/C but not of lamin B. Moreover, when compared with normal cells, neutrophils with the Pelger-Hu?t anomaly of nuclear hyposegmentation exhibited significantly lower activity of caspase-6, a lamin A/C-cleaving enzyme. Differentiated HL-60 cells showed higher activity of caspase-6 than that of untreated cells. These observations allow us to speculate that remodeling of nuclear lamins might underlie the mechanism for nuclear segmentation of neutrophils.     

Differential sensitivity of vimentin and nuclear lamins from Ehrlich ascites tumor cells toward Ca2+ -activated neutral thiol proteinase

A comparative study of the susceptibility of vimentin and nuclear lamins from cultured Ehrlich ascites tumor (EAT) cells to degradation by Ca2+ -activated neutral thiol proteinase (calpain) has been undertaken. While pure vimentin was degraded very quickly at physiological ionic strength by purified calpain, isolated lamin B was digested comparatively slowly and purified lamins A/C were fairly resistant to proteolytic degradation. Similar digestion patterns were obtained from vimentin and lamin B with intermediary breakdown products close in size to the corresponding alpha-helical rod domains. To exclude the possibility that the low susceptibility of isolated lamins to Ca2+-dependent proteolytic degradation was due to irreversible denaturation during their isolation and purification, Triton cytoskeletons were prepared and their nuclear lamina as well as vimentin filaments were exposed to relatively large quantities of purified calpain. Under these conditions, not only vimentin filaments but also lamins A and B were digested while lamin C remained intact to a high degree. The major breakdown products of vimentin and lamins were identified as polypeptides which were 35 to 45 amino acids longer than the corresponding alpha-helical rod domains. Most of the vimentin-derived material and all high molecular weight polypeptides originating from lamins remained associated with the Triton cytoskeletons as demonstrated by sodium dodecyl sulfate polyacrylamide gel electrophoresis in conjunction with immunoblotting. Indirect immunofluorescence and electron microscope analysis of the calpain-digested Triton cytoskeletons revealed that they still contained a laminalike structure around the nuclear chromatin and numerous structurally altered intermediate filaments in the cytoplasmic remnant, although all vimentin had been degraded with the formation of 40/41 kDa polypeptides as major digestion products. In untreated Triton cytoskeletons, the vimentin filaments seemed to be in direct physical contact with the nuclear lamina, whereas in digested Triton cytoskeletons there was a distinct gap between structurally altered filaments and the nuclear surface. This shows that vimentin filaments and the nuclear lamina are differentially susceptible to degradation by calpain under certain ionic conditions and suggests that both filamentous structures are intimately associated with each other.(ABSTRACT TRUNCATED AT 400 WORDS)     

Proteolytic activity associated with the nuclear scaffold. The effect of self-digestion on lamins

The inner aspect of the nuclear envelope is supported by a peripheral framework called the nuclear scaffold, which consists of both structural and functional proteins. Its major structural components, lamins A-C, form a highly polymerized and insoluble fibrous matrix during interphase of the cell cycle. Functional constituents of the scaffold include the 46-kDa nucleoside triphosphatase which is thought to participate in nucleocytoplasmic transport of mRNA. This 46-kDa component shares an amino-terminal sequence with lamins A and C, indicating that proteolytic remodeling of the nuclear scaffold may contribute to the generation of nucleoside triphosphatase activity (Clawson, G. A., Lackey, A., and T?kés, Z. A. (1988) Exp. Cell Res. 176, 180-186). We report here that neutral protease activity intimately associated with the nuclear scaffold is also a functional constituent. This activity has a considerable selectivity for lamins as shown by self-digestion of scaffold preparations, and it may participate in the remodeling of the nuclear scaffold after treatment with carcinogens.     

Mutations of phosphorylation sites in lamin A that prevent nuclear lamina disassembly in mitosis

The nuclear envelope is a dynamic structure that completely disassembles in response to MPF/cdc2 activity in mitosis. A key feature of this process is the hyperphosphorylation of the major structural proteins of the envelope, the nuclear lamins A, B, and C. Two highly conserved serine residues of the lamin protein (Ser-22 and Ser-392 of lamins A and C) are symmetrically positioned 5 amino acids from the ends of the large alpha-helical domain and are shown in the accompanying paper by Ward and Kirschner to be among four sites phosphorylated during nuclear envelope breakdown. Mutations in Ser-22 and Ser-392 that prevent phosphorylation at these sites block the disassembly of the nuclear lamina during mitosis. We propose a model for the regulation of lamin assembly in which phosphorylation just outside the ends of the alpha-helical domain controls the assembly dynamics of the lamin coiled-coil dimers.     

The role of CaaX-dependent modifications in membrane association of Xenopus nuclear lamin B3 during meiosis and the fate of B3 in transfected mitotic cells

Recent evidence shows that the COOH-terminal CaaX motif of lamins is necessary to target newly synthesized proteins to the nuclear envelope membranes. Isoprenylation at the CaaX-cysteine has been taken to explain the different fates of A- and B-type lamins during cell division. A-type lamins, which loose their isoprenylation shortly after incorporation into the lamina structure, become freely soluble upon mitotic nuclear envelope breakdown. Somatic B-type lamins, in contrast, are permanently isoprenylated and, although depolymerized during mitosis, remain associated with remnants of nuclear envelope membranes. However, Xenopus lamin B3, the major B-type lamin of amphibian oocytes and eggs, becomes soluble after nuclear envelope breakdown in meiotic metaphase. Here we show that Xenopus lamin B3 is permanently isoprenylated and carboxyl methylated in oocytes (interphase) and eggs (meiotic metaphase). When transfected into mouse L cells Xenopus lamin B3 is integrated into the host lamina and responds to cell cycle signals in a normal fashion. Notably, the ectopically expressed Xenopus lamin does not form heterooligomers with the endogenous lamins as revealed by a coprecipitation experiment with mitotic lamins. In contrast to the situation in amphibian eggs, a significant portion of lamin B3 remains associated with membranes during mitosis. We conclude from these data that the CaaX motif-mediated modifications, although necessary, are not sufficient for a stable association of lamins with membranes and that additional factors are involved in lamin-membrane binding.     

An amino-terminal domain of the hepatitis C virus NS3 protease is essential for interaction with NS4A.

Hepatitis C virus (HCV) genomic RNA is translated into a large polyprotein that is processed into structural and nonstructural proteins. Processing at the N termini of several nonstructural proteins requires sequences contained in both NS3 and NS4A. NS3 contains a serine protease, whereas the function of NS4A in proteolysis is yet to be determined. By using the vaccinia virus-T7 hybrid expression system to transiently express HCV polypeptides in HeLa cells, we studied the effect of several N-terminal and C-terminal deletions of HCV NS3 on the processing activity at all the downstream cleavage sites. In this way, we have delineated the minimal domain of NS3 required for the serine protease activity associated with this protein. In addition, we demonstrate the formation of a stable complex between NS3 and NS4A: analysis of the deletion mutants reveals a region at the N terminus of NS3 that is necessary for both complex formation and modulation of the proteolytic activity by NS4A but not for the NS4A-independent serine protease activity of NS3.     

Phosphorylation of nuclear proteins

Many nuclear proteins are phosphorylated: they range from enzymes to several structural proteins such as histones, non-histone chromosomal proteins and the nuclear lamins. The pattern of phosphorylation varies through the cell cycle. Although histone H1 is phosphorylated during interphase its phosphorylation increases sharply during mitosis. Histone H3, chromosomal protein HMG 14 and lamins A, B and C all show reversible phosphorylation during mitosis. Several nuclear kinases have been characterized, including one that increases during mitosis and phosphorylates H1 in vitro. Factors have been demonstrated in maturing amphibian oocytes and mitotic mammalian cells that induce chromosome condensation and breakdown of the nuclear membrane. The possibility that they are autocatalytic protein kinases is considered. The location of histone phosphorylation sites within the nucleosome is consistent with a role for phosphorylation in modulating chromatin folding.     

Functional and therapeutic analysis of hepatitis C virus NS3.4A protease control of antiviral immune defense

Chronic hepatitis C virus (HCV) infection is a major global public health problem. HCV infection is supported by viral strategies to evade the innate antiviral response wherein the viral NS3.4A protease complex targets and cleaves the interferon promoter stimulator-1 (IPS-1) adaptor protein to ablate signaling of interferon alpha/beta immune defenses. Here we examined the structural requirements of NS3.4A and the therapeutic potential of NS3.4A inhibitors to control the innate immune response against virus infection. The structural composition of NS3 includes an amino-terminal serine protease domain and a carboxyl-terminal RNA helicase domain. NS3 mutants lacking the helicase domain retained the ability to control virus signaling initiated by retinoic acid-inducible gene-I (RIG-I) or melanoma differentiation antigen 5 and suppressed the downstream activation of interferon regulatory factor-3 (IRF-3) and nuclear factor kappaB (NF-kappaB) through the targeted proteolysis of IPS-1. This regulation was abrogated by truncation of the NS3 protease domain or by point mutations that ablated protease activity. NS3.4A protease control of antiviral immune signaling was due to targeted proteolysis of IPS-1 by the NS3 protease domain and minimal NS4A cofactor. Treatment of HCV-infected cells with an NS3 protease inhibitor prevented IPS-1 proteolysis by the HCV protease and restored RIG-I immune defense signaling during infection. Thus, the NS3.4A protease domain can target IPS-1 for cleavage and is essential for blocking RIG-I signaling to IRF-3 and NF-kappaB, whereas the helicase domain is dispensable for this action. Our results indicate that NS3.4A protease inhibitors have immunomodulatory potential to restore innate immune defenses to HCV infection.     

Differential transport and integration into the nuclear lamina for lamins A, B, and C

Lamins A, B, and C are the major proteins of the mammalian nuclear lamina and have been well studied in BHK-21 cells. Using in vivo labelling, cell fractionation, and immunoprecipitation, we have found that lamins have different patterns of nuclear transport and solubility. Newly synthesized lamin A is translocated to the nucleus faster than lamin C or B. It is the most tightly bound lamin and cannot be extracted from the lamina by nonionic detergent or high-salt buffers. Lamins B and C migrate more slowly to the nucleus. Partitioning between cytoskeleton and detergent-soluble fractions shows that integration of lamins B and C is not completed before a 1-h chase. For lamin C this process is dependent upon protein synthesis and can be inhibited with cycloheximide. Even though lamins A and C are almost identical, lamin C is never firmly bound to the lamina and can be partially solubilized upon high-salt treatment.     

Head and/or CaaX domain deletions of lamin proteins disrupt preformed lamin A and C but not lamin B structure in mammalian cells

The nuclear lamina is an important determinant of nuclear architecture. Mutations in A-type but not B-type lamins cause a range of human genetic disorders, including muscular dystrophy. Dominant mutations in nuclear lamin proteins have been shown to disrupt a preformed lamina structure in Xenopus egg extracts. Here, a series of deletion mutations in lamins A and B1 were evaluated for their ability to disrupt lamina structure in Chinese hamster ovary cells. Deletions of either the lamin A "head" domain or the C-terminal CaaX domain formed intranuclear aggregates and resulted in the disruption of endogenous lamins A/C but not lamins B1/B2. By contrast, "head-less" lamin B1 localized to the nuclear rim with no detectable effect on endogenous lamins, whereas lamin B1 CaaX domain deletions formed intranuclear aggregates, disrupting endogenous lamins A/C but not lamins B1/B2. Filter binding assays revealed that a head/CaaX domain lamin B1 mutant interacted much more strongly with lamins A/C than with lamins B1/B2. Regulated induction of this mutant in stable cell lines resulted in the rapid elimination of all detectable lamin A protein, whereas lamin C was trapped in a soluble form within the intranuclear aggregates. In contrast to results in Xenopus egg extracts, dominant negative lamin B1 (but not lamin A) mutants trapped replication proteins involved in both the initiation and elongation phases of replication but did not effect cellular growth rates or the assembly of active replication centers. We conclude that elimination of the CaaX domain in lamin B1 and elimination of either the CaaX or head domain in lamin A constitute dominant mutations that can disrupt A-type but not B-type lamins, highlighting important differences in the way that A- and B-type lamins are integrated into the lamina.     

Angiotensin II stimulates phosphorylation of nuclear lamins via a protein kinase C-dependent mechanism in cultured vascular smooth muscle cells

Angiotensin II (ang II) induces c-fos gene expression in part via a protein kinase C-dependent mechanism in cultured vascular smooth muscle cells (VSMC). However, little is known about the mechanisms by which protein kinase C regulates nuclear functions. We examined the ability of ang II to phosphorylate nuclear lamina proteins in VSMC and the possibility that protein kinase C is involved in these putative phosphorylation events. Ang II stimulated the phosphorylation of Triton X-100- and high salt-insoluble nuclear envelope proteins with molecular weights of 70,000, 67,000, and 60,000. These proteins were identified as lamins A, B, and C, respectively, based on their mobilities on two-dimensional gel electrophoresis and interaction with antibodies to lamins as detected by immunoblot analyses. After a 2-min delay, phosphorylation levels of lamins increased, peaked at 20-30 min, and were sustained for at least 60 min after ang II stimulation. The threshold, half-maximal, and maximal concentrations of ang II which induced phosphorylation of lamins were 0.1, 0.5-1, and 100 nM, respectively. Phorbol 12-myristate 13-acetate also induced these reactions, whereas ionomycin did not. Down-regulation of protein kinase C by prolonged treatment with phorbol 12,13-dibutyrate attenuated ang II-induced phosphorylation of lamins. In vitro phosphorylation of nuclear envelope proteins by protein kinase C revealed that lamins served as substrates for this enzyme. These results indicate that ang II induces phosphorylation of lamins in cultured VSMC and suggest that protein kinase C is either directly or indirectly involved in these reactions. The results raise the possibility that phosphorylation of nuclear proteins is one of the important steps by which the protein kinase C signaling pathway regulates agonist-induced nuclear events.     

Characterisation of the role of zinc in the hepatitis C virus NS2/3 auto-cleavage and NS3 protease activities

Cleavage of the hepatitis C virus polyprotein between the non-structural NS2 and NS3 proteins is mediated by a poorly characterised auto-proteolytic activity that maps to the C terminus of NS2 and the N terminus of NS3, but is distinct from the NS3 protease activity responsible for downstream cleavages in the polyprotein. We have exploited the fact that the minimal precursor (residues 904-1206 of the HCV polyprotein) can be expressed as an insoluble protein in Escherichia coli and subsequently refolded into a form active for both auto-cleavage and NS3 protease activity, to further characterise the NS2/3 auto-cleavage activity. We show that both activities are zinc-dependent and show an absolute requirement for cysteine residues 1123, 1125 and 1171 within NS3. In contrast cysteine 922 (within NS2) is only required for NS2/3 auto-cleavage activity and histidine 1175 is only required for NS3 activity. Although the complete NS3 protease domain (including the C-terminal alpha-helix) is required for NS2/3 auto-cleavage, the activity of the NS3 protease is not essential. Lastly we show that the NS2/3 auto-cleavage activity is more sensitive to zinc chelation by 1,10-phenanthroline than the NS3 protease activity. This observation is consistent with different conformations of the precursor competent for either NS2/3 auto-cleavage or NS3 protease activity; these two conformations can be distinguished by their relative strength and geometry of zinc coordination.     

In vitro characterization of a purified NS2/3 protease variant of hepatitis C virus

The cleavage of the hepatitis C virus polyprotein between the nonstructural proteins NS2 and NS3 is mediated by the NS2/3 protease, whereas the NS3 protease is responsible for the cleavage of the downstream proteins. Purification and in vitro characterization of the NS2/3 protease has been hampered by its hydrophobic nature. NS2/3 protease activity could only be detected in cells or in in vitro translation assays with the addition of microsomal membranes or detergent. To facilitate purification of this poorly characterized protease, we truncated the N-terminal hydrophobic domain, resulting in an active enzyme with improved biophysical properties. We define a minimal catalytic region of NS2/3 protease retaining autocleavage activity that spans residues 904-1206 and includes the C-terminal half of NS2 and the N-terminal NS3 protease domain. The NS2/3 (904-1206) variant was purified from Escherichia coli inclusion bodies and refolded by gel filtration chromatography. The purified inactive form of NS2/3 (904-1206) was activated by the addition of glycerol and detergent to induce autocleavage at the predicted site between Leu(1026) and Ala(1027). NS2/3 (904-1206) activity was dependent on zinc ions and could be inhibited by NS4A peptides, peptides that span the cleavage site, or an N-terminal peptidic cleavage product. This NS2/3 variant will facilitate the development of an assay suitable for identifying inhibitors of HCV replication.