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Nuclear matrix constituent proteins (NMCPs), the structural components of the plant lamina, are considered to be the analogues of lamins in plants based on numerous structural and functional similarities. Current phylogenetic knowledge suggests that, in contrast to lamins, which are widely distributed in eukaryotes, NMCPs are taxonomically restricted to Streptophyta. At present, most information about NMCPs comes from angiosperms, and virtually no data are available from more ancestral groups. In angiosperms, the NMCP family comprises two phylogenetic groups, NMCP1 and NMCP2, which evolved from the NMCP1 and NMCP2 progenitor genes. Based on sequence conservation and the presence of NMCP-specific domains, we determined the structure and number of NMCP genes present in different Streptophyta clades. We analysed 91 species of embryophytes and report additional NMCP sequences from mosses, liverworts, clubmosses, horsetail, ferns, gymnosperms, and Charophyta algae. Our results confirm an origin of NMCPs in Charophyta (the earliest diverging group of Streptophyta), resolve the number and structure of NMCPs in the different clades, and propose the emergence of additional NMCP homologues by whole-genome duplication events. Immunofluorescence microscopy demonstrated localization of a basal NMCP from the moss Physcomitrella patens at the nuclear envelope, suggesting a functional conservation for basal and more evolved NMCPs., (© The Author(s) 2019. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

Nuclear lamina organization is similar in metazoan and plants though the latter lack orthologs of lamins, the main components of the metazoan lamina. Current evidence suggests that Nuclear Matrix Constituent Proteins (NMCPs) are the lamin analogues in plants as these proteins share several key features: higher-order secondary structure and domain layout, subnuclear distribution, and involvement in the regulation of nuclear shape and size, as well as in higher-order chromatin organization. Previously, we studied the NMCP family in flowering plants (angiosperms), in which it comprises two phylogenetic groups: NMCP1 and NMCP2. At present, in silico information about NMCP proteins in embryophytes is relatively advanced, though very few proteins, most of them of the NMCP1 type, have been extensively studied in vivo. We previously characterized the NCMP1 protein in the monocot Allium cepa. Here, we report the key features of a second protein of this species NMCP2, which presents a conserved sequence and domain layout. Immunofluorescence and immunoelectronmicroscopy evidence co-localization of endogenous AcNMCP2 and AcNMCP1 in the lamina, while Western blotting and immunoconfocal microscopy reveal a similar pattern of expression and distribution of both NMCP proteins in different root tissues. Our results provide novel insight about endogenous NMCP2-type proteins and complete the characterization of the NMCP family in A. cepa, thus advancing the current understanding of these structural proteins constituting the plant lamina.

Lamins are the main components of the metazoan lamina, and while the organization of the nuclear lamina of metazoans and plants is similar, there are apparently no genes encoding lamins or most lamin-binding proteins in plants. Thus, the plant lamina is not lamin-based and the proteins that form this structure are still to be characterized. Members of the plant NMCP/LINC/CRWN protein family share the typical tripartite structure of lamins, although the 2 exhibit no sequence similarity. However, given the many similarities between NMCP/LINC/CRWN proteins and lamins (structural organization, position of conserved regions, sub-nuclear distribution, solubility, and pattern of expression), these proteins are good candidates to carry out the functions of lamins in plants. Moreover, functional analysis of NMCP/LINC mutants has revealed their involvement in maintaining nuclear size and shape, another activity fulfilled by lamins. This review summarizes the current understanding of NMCP/LINC proteins and discusses future studies that will be required to demonstrate definitively that these proteins are plant analogs of lamins.