How plants LINC the SUN to KASH

Abstract

Linkers of the nucleoskeleton to the cytoskeleton (LINC) complexes formed by SUN and KASH proteins are conserved eukaryotic protein complexes that bridge the nuclear envelope (NE) via protein-protein interactions in the NE lumen. Revealed by opisthokont studies, LINC complexes are key players in multiple cellular processes, such as nuclear and chromosomal positioning and nuclear shape determination, which in turn influence the generation of gametes and several aspects of development. Although comparable processes have long been known in plants, the first plant nuclear envelope bridging complexes were only recently identified. WPP domain-interacting proteins at the outer NE have little homology to known opisthokont KASH proteins, but form complexes with SUN proteins at the inner NE that have plant-specific properties and functions. In this review, we will address the importance of LINC complex-regulated processes, describe the plant NE bridging complexes and compare them to opisthokont LINC complexes.

Keywords: Arabidopsis; KASH; LINC complex; SUN; nuclear envelope; plant.

Figure 1. Nuclear envelope bridging complexes in different organisms. Since SUN domains are relatively conserved across species, all SUN proteins are drawn as trimers according to the evidence provided for HsSUN2.^, For simplicity, all KASH proteins are drawn as monomers. See text for details.

Figure 2. Computed three-dimensional model of the SUN domain of AtSUN1. (A) The S251-D453 fragment of AtSUN1 was modeled using MODELER. The SUN domain of HsSUN2 was used as a template (PDB: 4FI9). Three models were computed and the one with the lowest zDOPE score is shown. Magenta, model of the AtSUN1 SUN domain. Cyan, SUN domain of HsSUN2. Gray, Nesprin-2 KASH domain in the HsSUN2-KASH complex. (B) Computed surface of the binding pocket for the KASH C-terminus in AtSUN1. Red, V301-N318 fragment of AtSUN1 corresponding to the KASH lid of HsSUN2 (Y567-S587). Orange, S324-C333 fragment of AtSUN1, corresponding to the cation-loop of HsSUN2 (Q593-C601). Purple, residue H360, S371, H439, and Y443 of AtSUN1, corresponding to H628, S641, Y703, Y707 of HsSUN2, respectively. Images were generated using UCSF Chimera package and POV-Ray (http://www.povray.org/).

Figure 3. Amino acid sequence alignment of plant SUN domains with the SUN domain of HsSUN2. Alignment was performed using ClustalW2 (http://www.ebi.ac.uk/Tools/msa/clustalw2/) with default settings, except that the output sequences were kept as input order. Image was generated using JalView and ClustalX color. GI, NCBI GenInfo identifier. Black frames in the alignment are numbered at the top. Frame 1 indicates C563 in HsSUN2, which forms a disulfide bond with Homo sapiens Nesprin-2 C6862. This disulfide bond is dispensable for SUN-KASH interaction, and in plant SUN domains this position is instead a conserved D or E. Frame 2 indicates the KASH-lid in HsSUN2, however, the sequences have low similarity between HsSUN2 and plant SUN proteins. Frame 3 represents the cation loop in HsSUN2. This cation loop and residues indicated by frame 4, 6, 7, 8 (correspond to H628, S641, Y703, and Y707 of HsSUN2, respectively) form the pocket holding the KASH C-terminus and are well conserved in plant SUN domains. Frame 5 represents N636 of HsSUN2, the N-glycosylation site. N-glycosylation of HsSUN2 is dispensable for KASH binding, and this position is a conserved D in plant SUN domains.