Unravelling ER Morphology: Meet Arabidopsis’ Lunapark Proteins

Microscopy Plant Biology

Inside every plant cell lies a complex, constantly moving structure known as the endoplasmic reticulum (ER). This organelle plays a central role in cellular health by supporting protein and lipid production, regulating calcium levels, and responding to environmental stress. The ER is shaped as a network of interconnected tubules and flat membrane sheets called cisternae, but the molecular machinery that maintains this structure in plants is still being uncovered.

In this study, we identified and characterized two proteins in Arabidopsis thaliana, AtLNP1 and AtLNP2, which are homologous to a protein family called Lunaparks (LNPs) known in yeast and mammalian cells. These proteins are thought to shape the ER’s architecture. Using advanced fluorescence microscopy and genetic tools, we found that AtLNP1 primarily localizes to ER cisternae, while AtLNP2 is found throughout the ER, including both cisternae and tubules. Overexpression of either protein led to more cisternae, whereas knockdown mutants displayed fewer cisternae and a more open, less structured ER network.

Unlike their animal and yeast counterparts, which typically stabilize three-way junctions in the ER, plant LNP proteins appear to function differently. We observed that AtLNP1 and AtLNP2 do not strongly localize to these junctions and do not significantly influence their stability. Instead, loss of these proteins leads to a significant reduction in cisternae, suggesting that in plants, LNP proteins primarily regulate cisternal formation or stability. This is a key difference from other systems, where LNPs are implicated in shaping tubular networks and controlling junction density.

Interestingly, we also showed that AtLNP proteins physically interact with another family of ER-shaping proteins called reticulons, which typically localize to the highly curved edges of membranes. Co-expression studies revealed that reticulons can suppress the cisternal expansion caused by LNP overexpression, hinting at a collaborative mechanism: LNPs may promote flat membrane formation, while reticulons prevent excessive expansion by curving the edges. Together, they help maintain the delicate balance between tubules and cisternae in the ER network.

These findings provide a significant step forward in understanding how plant cells build and maintain the structure of their ER. The presence of two distinct yet partially redundant LNP proteins in Arabidopsis—a feature not seen in yeast or mammals—raises interesting questions about how plants may have evolved unique mechanisms for ER organization. Ongoing work in our lab is now focused on dissecting the interactions between LNPs and other ER morphogens, which will help reveal how ER architecture supports the broader physiological roles of this essential organelle in plant development and stress resilience.