A lamin in lower eukaryotes?

Abstract

Lamins are the major components of the nuclear lamina and serve not only as a mechanical support, but are also involved in chromatin organization, epigenetic regulation, transcription and mitotic events. Despite these universal tasks, lamins have so far been found only in metazoans. Yet, recently we have identified Dictyostelium NE81 as the first lamin-like protein in a lower eukaryote. Based on the current knowledge, we draw a model for nuclear envelope organization in Dictyostelium in this Extra View and we review the experimental data that justified this classification. Furthermore we provide unpublished data underscoring the requirement of posttranslational CaaX-box processing for proper protein localization at the nuclear envelope. Sequence comparison of NE81 sequences from four Dictyostelia with bona fide lamins illustrates the evolutional relationship between these proteins. Under certain conditions these usually unicellular social amoebae congregate to form a multicellular body. We propose that the evolution of the lamin-like NE81 went along with the invention of multicellularity.

Figure 1. Phenotypes of NE81 mutant cells. Immunofluorescence microscopy of icmA null cells (A), GFP-NE81SLIM cells (B), NE81 null cells (C), GFP-NE81 overexpressing cells (D) and control cells (E) fixed with glutaraldehyde and stained with anti-NE81/anti-rabbit-AlexaFluor 488 and anti-tubulin YL1/2,/anti-rat-AlexaFluor 568. Mitotic stages in (B) are indicated. The montage shows GFP fluorescence in green, tubulin in red and nuclei in blue. Maximum intensity projections of widefield deconvolution image stacks are shown. Bar = 2.5 μm.

Figure 2. Hypothetical model of nuclear envelope organization in the pericentrosomal region in Dictyostelium. In both nuclear membranes (black line) Sun1 is involved in linkage of the centrosome to an NE81-based nuclear lamina. On the centrosomal side, the centrosomal corona protein CP148 is directly or indirectly associated with Sun1, on the nuclear side Sun1 is directly or indirectly bound to NE81. Centromere proteins are also associated with Sun1 and mediate clustering of centromeres in the pericentrosomal region. The tight linkage between the cytosolic centrosome and the centromere cluster and nuclear lamina could be mediated by a self-interaction between SUN-domains or by an unknown protein within the perinuclear space. At the outer nuclear membrane both the kinesin Kif9 and Sun1-associated dynein/dynactin bind to microtubules and help to hold the centrosome close to the nucleus.

Figure 3. Protein sequence alignment of two bona fide lamins with NE81 orthologs from four Dictyostelidae. Coding sequences from the social amoebae D. discoideum (starting with aa position 99), D. purpureum (starting with aa position 84), D. fasciculatum (starting with aa position 110) and Polysphondylium pallidum (starting with aa position 119) and Amphimedon queenslandica were derived from the respective genome projects (dictybase.org, sacgb.fli-leibniz.de, www.metazome.net/amphimedon). In case of P. pallidum the predicted second intron had to be neglected since otherwise the deduced amino acid sequence were devoid of a CaaX-box. Sequences were aligned using MultAlin software. High consensus of amino acid similarity is colored in red, low consensus in blue. Heptad repeats of the human B1 are marked at the 1st and 4th position (a and d positions) below the sequence. Filled circles indicate hydrophobic and uncharged amino acid residues favorable for coiled-coil formation (L, I, V, A, M, N, Q, Y, F and W), open circles indicate less favorable amino acid residues (E, D, K, R, H, S, T, C and G). Deduced from the lamin heptad pattern, tentative heptad positions were assigned to the NE81 sequences (above the sequence). A small linker, marked by a bracket with a (?), had to be introduced into the NE81 sequence to match the heptad pattern of the lamins. Linker regions separating the coils of lamins are marked by brackets. The discontinuity in the heptad pattern in the second half of coil 2B (stutter) is indicated by an arrow above and below the D. discoideum and human lamin B1 sequences, respectively. The region of the Ig-domain within the tail of the lamin sequences is indicated by a double-headed arrow. Three characteristic sequence motifs for lamins are indicated: a CDK1 phosphorylation consensus sequence upstream of the rod domain (*), a putative nuclear localization sequence (####), and the C-terminal CaaX-box (in bold letters). In case of D. fasciculatum the CDK1 target sequence does not match with the consensus (S/T-P-X-R/K).