Assembly of Drosophila lamin Dm0 and C mutant proteins studied with the baculovirus system

Invertebrate lamins

Lamins are the main component of the nuclear lamina and considered to be the ancestors of all intermediate filament proteins. They are localized mainly at the nuclear periphery where they form protein complexes with integral proteins of the nuclear inner membrane, transcriptional regulators, histones and chromatin modifiers. Studying lamins in invertebrate species has unique advantages including the smaller number of lamin genes in the invertebrate genomes and powerful genetic analyses in Caenorhabditis elegans and Drosophila melanogaster. These simpler nuclear lamina systems allow direct analyses of their structure and functions. Here we give an overview of recent advances in the field of invertebrate nuclear lamins with special emphasis on their evolution, assembly and functions.

Solubility properties and specific assembly pathways of the B-type lamin from Caenorhabditis elegans

Lamins are nucleus-specific intermediate filament (IF) proteins that together with a complex set of membrane proteins form a filamentous meshwork tightly adhering to the inner nuclear membrane and being associated with the nuclear pore complexes. This so-called nuclear lamina provides mechanical stability and, in addition, has been implicated in the spatial organization of the heterochromatin. While increasing knowledge on the biological function of lamins has been obtained in recent years, the assembly mechanism of lamin filaments at the molecular level has remained largely elusive. Therefore, we have now more systematically investigated lamin assembly in vitro. Using Caenorhabditis elegans lamin, which has been reported to assemble into 10-nm filaments under low ionic strength conditions, we investigated the assembly kinetics of this protein into filaments in more detail using both His-tagged and un-tagged recombinant proteins. In particular, we have characterized distinct intermediates in the filament assembly process by analytical ultracentrifugation, electron and atomic force microscopy. In contrast to the general view that lamins assemble only slowly into filaments, we show that in vitro association reactions are extremely fast, and depending on the ionic conditions employed, significant filamentous assemblies form within seconds.

The lamin CxxM motif promotes nuclear membrane growth

We analyzed the influence of lamins on nuclear envelope growth in cultured Xenopus A6 cells by the overexpression of human lamin A, Xenopus and zebrafish lamins B2 and Drosophila lamins Dm0 and C as GFP fusion proteins. Lamins containing a CxxM motif in their primary sequence (lamins A, B2, Dm0) induced the formation of lobulated nuclei with multi-membrane-layered, highly folded nuclear membranes and intranuclear membrane assemblies, as observed by electron microscopy. Such morphological alterations were not observed with Drosophila lamin C, a lamin without this motif or with a lamin B2 mutant (B2-SxxM) where the cysteine of the CxxM motif is replaced by a serine. Drosophila lamin C mutants containing a CxxM motif behaved like B-type lamins thus confirming that this tetrapeptide is directly involved in the morphological changes we observed. Nuclear membrane proliferation could also be induced by lamin B2 in COS-7 cells and in zebrafish embryos but not by human lamin A in COS-7 cells. We speculate that the human lamin A is incompletely processed in Xenopus A6 cells and therefore behaves in this cell line like a B-type lamin. Our results indicate that the CxxM motif of B-type lamins has a dual function: it mediates lamin targeting to the inner nuclear membrane thereby promoting nuclear membrane growth.

The lamin B receptor of Drosophila melanogaster

The lamin B receptor (LBR) is an integral membrane protein of the inner nuclear membrane that has so far been characterized only in vertebrates. Here, we describe the Drosophila melanogaster protein encoded by the annotated gene CG17952 that is the putative ortholog to the vertebrate LBR. The Drosophila lamin B receptor (dLBR) has the following properties in common with the vertebrate LBR. First, structure predictions indicate that the 741 amino acid dLBR protein possesses a highly charged N-terminal domain of 307 amino acids followed by eight transmembrane segments in the C-terminal domain of the molecule. Second, immunolocalization and cell fractionation reveal that the dLBR is an integral membrane protein of the inner nuclear membrane. Third, dLBR can be shown by co-immunoprecipitations and in vitro binding assays to bind to the Drosophila B-type lamin Dm0. Fourth, the N-terminal domain of dLBR is sufficient for in vitro binding to sperm chromatin and lamin Dm0. In contrast to the human LBR, dLBR does not possess sterol C14 reductase activity when it is expressed in the Saccharomyces cerevisiae erg24 mutant, which lacks sterol C14 reductase activity. Our data raise the possibility that, during evolution, the enzymatic activity of this insect protein had been lost. To determine whether the dLBR is an essential protein, we depleted it by RNA interference in Drosophila embryos and in cultured S2 and Kc167 cells. There is no obvious effect on the nuclear architecture or viability of treated cells and embryos, whereas the depletion of Drosophila lamin Dm0 in cultured cells and embryos caused morphological alterations of nuclei, nuclear fragility and the arrest of embryonic development. We conclude that dLBR is not a limiting component of the nuclear architecture in Drosophila cells during the first 2 days of development.

The nuclear lamina and its functions in the nucleus

The nuclear lamina is a structure near the inner nuclear membrane and the peripheral chromatin. It is composed of lamins, which are also present in the nuclear interior, and lamin-associated proteins. The increasing number of proteins that interact with lamins and the compound interactions between these proteins and chromatin-associated proteins make the nuclear lamina a highly complex but also a very exciting structure. The nuclear lamina is an essential component of metazoan cells. It is involved in most nuclear activities including DNA replication, RNA transcription, nuclear and chromatin organization, cell cycle regulation, cell development and differentiation, nuclear migration, and apoptosis. Specific mutations in nuclear lamina genes cause a wide range of heritable human diseases. These diseases include Emery-Dreifuss muscular dystrophy, limb girdle muscular dystrophy, dilated cardiomyopathy (DCM) with conduction system disease, familial partial lipodystrophy (FPLD), autosomal recessive axonal neuropathy (Charcot-Marie-Tooth disorder type 2, CMT2), mandibuloacral dysplasia (MAD), Hutchison Gilford Progeria syndrome (HGS), Greenberg Skeletal Dysplasia, and Pelger-Huet anomaly (PHA). Genetic analyses in Caenorhabditis elegans, Drosophila, and mice show new insights into the functions of the nuclear lamina, and recent structural analyses have begun to unravel the molecular structure and assembly of lamins and their associated proteins.

Two novel LEM-domain proteins are splice products of the annotated Drosophila melanogaster gene CG9424 (Bocksbeutel)

The LEM motif is a sequence of 40-50 amino acids that has been identified in a number of non-related proteins of the inner nuclear membrane including the lamina-associated polypeptides 2 (LAP2), emerin, MAN1 and the Drosophila protein otefin. This evolutionary conserved sequence motif can mediate via the interaction with the small protein BAF the binding of LEM-domain proteins to DNA. Taking advantage of its sequenced genome we analyzed whether Drosophila possesses beside otefin additional genes coding for proteins with a LEM motif. A putative candidate gene was the annotated gene CG9424 which we named Bocksbeutel. Of all putative Drosophila LEM-domain proteins, otefin and Bocksbeutel exhibited the highest similarity in the LEM motif (53% identical amino acids). The Bocksbeutel gene can code for two isoforms of 399 and 351 amino acids that are produced by alternative splicing. In the alpha-isoform a transmembrane domain is localized close to the carboxyterminus. This segment is absent in the shorter beta-isoform. By RT-PCR we could show that in the embryo the mRNA coding for the alpha-isoform and in significantly lower amounts the mRNA coding for the beta-isoform are expressed. When expressed in transfected cells as GFP fusion proteins, the beta-isoform is localized predominantly in the nucleoplasm and the alpha-isoform is targeted to the nuclear envelope, indicating that Bocksbeutel-alpha is localized in the inner nuclear membrane. Bocksbeutel-alpha is the predominant isoform expressed in cells, larvae, and flies. Indirect immunofluorescence with Bocksbeutel-specific antibodies on tissues and cultured cells revealed that Bocksbeutel proteins are localized in the nuclear envelope and in the cytoplasm. By RNA interference we have down-regulated the expression of Bocksbeutel, BAF, otefin, and lamin DmO in Drosophila Kc167 cells. The down-regulation of Bocksbeutel and otefin had no influence on the viability of Kc167 cells and the intracellular localization of all other nuclear and nuclear envelope proteins analyzed. In contrast, when lamin DmO was reduced by RNAi the distribution of Bocksbeutel and otefin in the nuclear envelope of Kc167 cells was significantly altered. We conclude that the two LEM-domain proteins Bocksbeutel and otefin are no limiting components for the maintenance of the nuclear architecture in cultured Drosophila cells at interphase.