Telomeres, tethers and trypanosomes

Abstract

Temporal and spatial organization of the nucleus is critical for the control of transcription, mRNA processing and the assembly of ribosomes. This includes the occupancy of specific territories by mammalian chromosomes, the presence of subnuclear compartments such as the nucleolus and Cajal bodies and the division of chromatin between active and inactive states. These latter are commonly associated with the location of DNA within euchromatin and heterochromatin respectively; critically these distinctions arise through modifications to chromatin-associated proteins, including histones, as well as the preferential localization of heterochromatin at the nuclear periphery. Most research on nuclear organization has focused on metazoa and fungi; however, recent technical advances have made more divergent eukaryotes accessible to study, with some surprising results. For example, the organization of heterochromatin is mediated in metazoan nuclei in large part by lamins, the prototypical intermediate filament proteins. Despite the presence of heterochromatin, detected both biochemically and by EM in most eukaryotic organisms, until this year lamins were thought to be restricted to metazoan taxa, and the proteins comprising the lamina in other lineages were unknown. Recent work indicates the presence of lamin orthologs in amoeba, while trypanosomatids possess a large coiled-coil protein, NUP-1, that performs functions analogous to lamins. These data indicate that the presence of a nuclear lamina is substantially more widespread than previously thought, with major implications for the evolution of eukaryotic gene expression mechanisms. We discuss these and other recent findings on the organization of nuclei in diverse organisms, and the implications of these findings for the evolutionary origin of eukaryotes.

Structural insights into LINC complexes

“…In contrast, the three additional genes coding for Sun3, Sun4, and Sun5, respectively, all show a much more restricted, testis-specific expression pattern [23]. In addition to the SUN domains at the C terminus, there are also SUN-like proteins [24,25] with the SUN-like domain in the center of the protein. To what extent SUN- and SUN-like proteins are functionally related is currently a matter of speculation.…”

LINCing the eukaryotic tree of life – towards a broad evolutionary comparison of nucleocytoplasmic bridging complexes

“…Trypanosoma brucei belongs to the supergroup Excavata, and is thus as distantly related to the opisthokonts as the Archaeplastida (Field et al, 2012). The nuclear pore complex and the karyopherins are conserved, but there is no evidence for LINC complexes.…”

“…The nuclear pore complex and the karyopherins are conserved, but there is no evidence for LINC complexes. There are, however, SUN-like proteins encoded in other Excavata lineages, such as diplomonads and heterolobosids, and it is thus possible that the trypanosomatids have selectively lost LINC complexes (Field et al, 2012). Identifying Excavata KASH-like proteins would be extremely helpful in broadening the current data set on possibly ancestral LINC complexes.…”

“…According to the most current phylogenetic tree, extant eucaryotes are grouped into five supergroups, with the Amoebozoa and the Opisthokonta together forming the unikonts (one flagellum) and the Archaeplastida, SAR-CCTH (Stramenopile, Alveolata, Rhizaria and Cryptophyta, Centrohelida, Telonemia and Haplophyta) and the Excavata forming the bikonts (two flagella) (Adl et al, 2012;Field et al, 2012;Gräf et al, 2015) (Fig. 4) bikont group members for which we have any information regarding SUN and KASH protein structure and function.…”

“…The current eukaryotic evolutionary tree places all organisms between yeast and man into the supergroup Opisthokonta (Adl et al, 2012;Field et al, 2012;Gräf et al, 2015). Almost all information available on LINC complexes is derived from research on model organisms within this supergroup, with the 'canonical' LINC complexes being those that connect the lamina to the three types of cytoskeletal filaments in the cytoplasm.…”