The Strange Case of the Noncanonical Lamina: Deep Divisions in Nuclear Organisation?

The nuclear envelope is subtended in most eukaryotes by a proteinaceous lamina, a network that has been recognised since the 1950s. Originally considered as a simple structural support, it is now clear that the lamina can be highly dynamic and participates in a multitude of functions, including transcriptional and epigenetic regulation, definition of chromatin domains, genome stability and the positioning of nuclear pore complexes. The major protein components of the lamina in metazoans are ~60 kDa lamins, which assemble to form fibres and a network and are regulated by multiple mechanisms. Despite a broad taxonomic distribution beyond Metazoa, lamins are not universal and, in at least three major lineages, are absent, specifically fungi, plants and kinetoplastid protists. The latter two possess lineage-specific lamin analogues, the CRWN and NUP-1/NUP-2 proteins, respectively. Here we discuss and compare the kinetoplastid and plant lamina, their origins, components and functions and spectacular examples of convergent evolution.