SEED LIPID DROPLET PROTEIN1, SEED LIPID DROPLET PROTEIN2, and LIPID DROPLET PLASMA MEMBRANE ADAPTOR mediate lipid droplet–plasma membrane tethering

Abstract

Membrane contact sites (MCSs) are inter-organellar connections that allow for the direct exchange of molecules, such as lipids or Ca2+ between organelles, but can also serve to tether organelles at specific locations within cells. Here we identified and characterised three proteins of Arabidopsis thaliana that form a lipid droplet (LD)-plasma membrane (PM) tethering complex in plant cells, namely LD-localised SEED LD PROTEIN (SLDP) 1 and SLDP2 and PM-localised LD-PLASMA MEMBRANE ADAPTOR (LIPA). Using proteomics and different protein–protein interaction assays, we show that both SLDPs associate with LIPA. Disruption of either SLDP1 and SLDP2 expression, or that of LIPA, leads to an aberrant clustering of LDs in Arabidopsis seedlings. Ectopic co-expression of one of the SLDPs with LIPA is sufficient to reconstitute LD-PM tethering in Nicotiana tabacum pollen tubes, a cell type characterised by dynamically moving LDs in the cytosolic streaming. Further, confocal laser scanning microscopy revealed both SLDP2.1 and LIPA to be enriched at LD–PM contact sites in seedlings. These and other results suggest that SLDP and LIPA interact to form a tethering complex that anchors a subset of LDs to the PM during post-germinative seedling growth in Arabidopsis.

A seed‐like proteome in oil‐rich tubers

“…Two steroleosin isoforms were detected in tubers (Figure S5b). Interestingly, seed lipid droplet protein (SLDP), another LD‐associated protein (Kretzschmar et al., 2020) that is seed‐ and seedling‐specific in Arabidopsis (Klepikova et al., 2016) and is part of a membrane contact site between LDs and the plasma membrane (Krawczyk et al., 2022; Scholz et al., 2022), was also detected in tubers.…”

Plasticity of the Arabidopsis leaf lipidome and proteome in response to pathogen infection and heat stress

“…prompting the formation of membrane contact sites for lipid remodeling. A first such contact site of LDs with the plasma membrane has been recently described (Krawczyk et al, 2022b) and although the physiological relevance is unclear as of yet, the interaction of LDs in plant membrane contact sites especially in reaction to stress are an exciting avenue of future LD research.…”

Finding new friends and revisiting old ones – how plant lipid droplets connect with other subcellular structures

“…Notably, in the last few years, many new LD proteins have been discovered that are directly involved in LD biogenesis, function, and/or breakdown (Horn et al ., 2013; Pyc et al ., 2017, 2021; Fernandez‐Santos et al ., 2020; Kretzschmar et al ., 2020). Some of these proteins have been also implicated in mediating the physical connections that exist between LDs and other subcellular structures (Greer et al ., 2020; Pyc et al ., 2021; Krawczyk et al ., 2022b). These connections are defined as membrane contact sites (MCSs) if the following conditions are fulfilled: (1) a connection of organelles mediated by tethering proteins, (2) no fusion of the organelles, (3) the fulfillment of a specific function(s) by the MCS, and (4) the presence of a defined proteome at the MCS (Scorrano et al ., 2019).…”

“…Structurally, LDs are relatively simple, in that their core consists of TAG, SE and/or other hydrophobic substances that are partitioned from the aqueous cytosol by a phospholipid monolayer (Pyc et al ., 2017). Furthermore, various proteins are integrated into the LD monolayer, some of which are thought to extend into the hydrophobic core (Huang, 2018), while others appear to be attached directly to the LD surface (Gidda et al ., 2016) or through protein–protein interactions (Krawczyk et al ., 2022b). Notably, in the last few years, many new LD proteins have been discovered that are directly involved in LD biogenesis, function, and/or breakdown (Horn et al ., 2013; Pyc et al ., 2017, 2021; Fernandez‐Santos et al ., 2020; Kretzschmar et al ., 2020).…”

“…During seed germination and seedling establishment of Arabidopsis, LDs are generally closely associated with the plasma membrane (Kretzschmar et al ., 2018). While the reason for this could be that they are simply appressed to the plasma membrane due to the large central vacuole, recent studies have shown that three proteins are specifically involved in the formation of this association (Krawczyk et al ., 2022b). Two of these proteins involved in LD‐plasma membrane connections in Arabidopsis are the closely related SEED LIPID DROPLET PROTEIN 1 and 2 (SLDP1 and 2), which bind to LDs via a hydrophobic region near their N‐termini and can also directly interact with the LIPID DROPLET PLASMA MEMBRANE ADAPTOR (LIPA) protein localized at the plasma membrane, thereby forming an MCS (Fig.…”