Arabidopsis Lunapark proteins are involved in ER cisternae formation

Abstract

Summary The plant endoplasmic reticulum (ER) is crucial to the maintenance of cellular homeostasis. The ER consists of a dynamic and continuously remodelling network of tubules and cisternae. Several conserved membrane proteins have been implicated in formation and maintenance of the ER network in plants, such as RHD3 and the reticulon proteins. Despite the recent work in mammalian and yeast cells, the detailed molecular mechanisms of ER network organization in plants remain largely unknown. Recently, novel ER network‐shaping proteins called Lunapark (LNP) have been identified in yeast and mammalian cells.Here we identify two Arabidopsis LNP homologues and investigate their subcellular localization via confocal microscopy and potential function in shaping the ER network using protein–protein interaction assays and mutant analysis.We show that AtLNP1 overexpression in tobacco leaf epidermal cells mainly labels cisternae in the ER network, whereas AtLNP2 labels the whole ER. Overexpression of LNP proteins results in an increased abundance of ER cisternae and lnp1 and lnp1lnp2 amiRNA lines display a reduction in cisternae and larger polygonal areas.Thus, we hypothesize that AtLNP1 and AtLNP2 are involved in determining the network morphology of the plant ER, possibly by regulating the formation of ER cisternae.

Quantitative analysis of plant ER architecture and dynamics

“…Members of the reticulon family are also required along the periphery of the cisternae to induce curvature 9,38,60 , while other proteins, such as Arabidopsis Lunapark 1 and 2, are suggested to maintain or induce the sheet-like structure 19 . Previous descriptions of protein localisation have been essentially qualitative, but these distributions can also be quantified.…”

“…For example, over-expression of members of the reticulon ER-shaping protein family 14–18 , causes constrictions along the length of the tubules, and can convert cisternae to tubules 16 . Likewise, the morphology of the cisternae is affected by proteins, such as Lunapark 1 and 2, that can induce cisterna formation 19 , and drug treatments, such as Brefeldin A (BFA) 13 , which can vary the size and abundance of cisternae. The cisternae may not be simple uniform compartments.…”

Reciprocal regulation between lunapark and atlastin facilitates ER three-way junction formation

“…In mammalian cells, loss of Lnp results in sheet expansion (Shemesh et al, 2014 ; Chen et al, 2015 ; Wang et al, 2016 ) and Lnp-enriched junctions are more stable (Chen et al, 2015 ; Wang et al, 2016 ). In plant cells, two homologues of Lnp were identified, which are involved in determining the ER network morphology by regulating the formation of ER cisternae (Kriechbaumer et al, 2018 ). Lnp possesses two closely spaced transmembrane (TM) segments, forming a TM hairpin (TMH) structure that likely prefers curved membranes, such as ER tubules.…”

A Once-Hidden Endoplasmic Reticulum Matrix Reveals the Totally Tubular Function of LUNAPARKs in Plants

“…Compared to the wild type, lnp mutants display cortical ER that seems to have fewer sheets and larger intertubule spaces (lacunae), whereas overexpression of Arabidopsis (Arabidopsis thaliana) LNP1 or LNP2 in tobacco (Nicotiana tabacum) causes an increase in ER sheets. It was therefore hypothesized that LNPs raise the ratio of sheets to tubules (Kriechbaumer et al, 2018), a role opposite to that of the animal and yeast homologs. Additionally, because of the presence of large clumps of ER membrane in lnp mutants, it was hypothesized that LNPs play a role in the proper distribution of ER throughout the cell (Ueda et al, 2018).…”