8.A.102. The Reticulon (Reticulon) Family
Primary plant plasmodesmata (PD) arise at cytokinesis when the new cell plate forms. During this process, fine strands of endoplasmic reticulum (ER) are laid down between enlarging Golgi-derived vesicles to form nascent PD, each pore containing a desmotubule, a membranous rod derived from the cortical ER. Members of the reticulon (RTNLB) family of ER-tubulating proteins in Arabidopsis thaliana play a role in the formation of the desmotubule. RTNLB3 and RTNLB6, two RTNLBs present in the PD proteome, are recruited to the cell plate at late telophase, when primary PD are formed, and remain associated with primary PD in the mature cell wall (Knox et al. 2015). Both RTNLBs show significant colocalization at PD. These RTNLBs are mobile at the edge of the developing cell plate where new wall materials are being delivered, but significantly less mobile at its center, where PD are forming. Reticulons are involved in peroxysomal biogenesis (Mast et al. 2016). Reticulon proteins regulate intracellular trafficing in plants (Lee et al. 2011). RTN3 is directly involved in the ER-constituent trafficking events through dually acting as an essential and important ER-stress sensor, and a trigger for Bcl-2 translocation (Wan et al. 2007).
Reticulons are within a large family of integral membrane proteins that are ubiquitous in eukaryotes and play a key role in functional remodelling of the endoplasmic reticulum membrane. The reticulon family is especially large in plants, with the Arabidopsis thaliana genome containing twenty-one isoforms. Reticulons vary in length but all contain a conserved C-terminal reticulon homology domain (RHD) that associates with membranes. The structure of one small homologue has been modeled (Chow et al. 2018).