The DEK1 Calpain Linker Functions in Three-Dimensional Body Patterning in Physcomitrella patens

The DEFECTIVE KERNEL1 (DEK1) calpain is a conserved 240-kD key regulator of three-dimensional body patterning in land plants acting via mitotic cell plane positioning. The activity of the cytosolic C-terminal calpain protease is regulated by the membrane-anchored DEK1 MEM, which is connected to the calpain via the 600-amino acid residue Linker. Similar to the calpain and MEM domains, the Linker is highly conserved in the land plant lineage, the similarity dropping sharply compared with orthologous charophyte sequences. Using site-directed mutagenesis, we studied the effect on Physcomitrella patens development by deleting the Linker and two conserved Linker motifs. The results show that removal of the Linker has nearly the same effect as removal of the entire DEK1 gene. In contrast, deletion of the conserved Laminin_G3 (LG3) domain had a milder effect, perturbing leafy gametophore patterning and archegonia development. The LG3 domain from Marchantia polymorpha is fully functional in P. patens, whereas angiosperm sequences are not functional. Deletion of a C-terminal Linker subsegment containing a potential calpain autolytic site severely disturbs gametophore development. Finally, changing one of the three calpain active-site amino acid residues results in the same phenotype as deleting the entire DEK1 gene. Based on the conserved nature of animal and DEK1 calpains, we propose that the DEK1 MEM-Linker complex inactivates the calpain by forcing apart the two calpain subunits carrying the three amino acids of the active site.