Lipid Droplet-Associated Proteins (LDAPs) Are Required for the Dynamic Regulation of Neutral Lipid Compartmentation in Plant Cells

Abstract

Eukaryotic cells compartmentalize neutral lipids into organelles called lipid droplets (LDs), and while much is known about the role of LDs in storing triacylglycerols in seeds, their biogenesis and function in nonseed tissues are poorly understood. Recently, we identified a class of plant-specific, lipid droplet-associated proteins (LDAPs) that are abundant components of LDs in nonseed cell types. Here, we characterized the three LDAPs in Arabidopsis (Arabidopsis thaliana) to gain insight to their targeting, assembly, and influence on LD function and dynamics. While all three LDAPs targeted specifically to the LD surface, truncation analysis of LDAP3 revealed that essentially the entire protein was required for LD localization. The association of LDAP3 with LDs was detergent sensitive, but the protein bound with similar affinity to synthetic liposomes of various phospholipid compositions, suggesting that other factors contributed to targeting specificity. Investigation of LD dynamics in leaves revealed that LD abundance was modulated during the diurnal cycle, and characterization of LDAP misexpression mutants indicated that all three LDAPs were important for this process. LD abundance was increased significantly during abiotic stress, and characterization of mutant lines revealed that LDAP1 and LDAP3 were required for the proper induction of LDs during heat and cold temperature stress, respectively. Furthermore, LDAP1 was required for proper neutral lipid compartmentalization and triacylglycerol degradation during postgerminative growth. Taken together, these studies reveal that LDAPs are required for the maintenance and regulation of LDs in plant cells and perform nonredundant functions in various physiological contexts, including stress response and postgerminative growth.

Membrane Dynamics and Multiple Functions of Oil Bodies in Seeds and Leaves

“…Leaf oil bodies contain the following proteins: caleosins, a-dioxygenase, and lipid droplet-associated proteins (LDAPs; Fig. 5A; Shimada et al, 2014;Gidda et al, 2016). Genes involved in oleosin biosynthesis are not expressed in leaves (Kim et al, 2002), suggesting that leaf oil body components differ from those of seed oil bodies.…”

“…The structure of rubber particles is similar to that of oil bodies. The Arabidopsis genome contains three LDAP/SRPP genes: LDAP1/SRP1, LDAP2/SRP2, and LDAP3/SRP3 (Gidda et al, 2016;Kim et al, 2016a). Proteomic analyses of leaf oil bodies in senescent Arabidopsis leaves indicate that LDAP1 is localized on oil bodies ( Fig.…”

“…5A; Brocard et al, 2017). LDAP overexpression increases the abundance of dark-induced leaf oil bodies, whereas LDAP deficiency reduces their abundance (Gidda et al, 2016). Oil bodies in LDAP-overexpressing seedling hypocotyls are larger than those in wild-type seedlings, and LDAPoverexpressing plants have longer roots and are more resistant to drought stress than wild-type plants (Kim et al, 2016a).…”

“…H. brasiliensis cis-prenyltransferases (HRTs) interact with HRT1-REF bridging protein, which binds REF and HRT for efficient rubber biosynthesis (Yamashita et al, 2016). LDAPs may recruit a complex for lipid biosynthesis, although they do not interact with seed oil bodies containing oleosins (Gidda et al, 2016), suggesting that LDAPs are antagonistic to oleosins in oil body targeting. LDAP is a unique protein involved in the physiological function of leaf oil bodies.…”

“…1; . Environmental conditions such as dark and extreme temperature (heat and cold) induce oil body formation (Gidda et al, 2016), suggesting that leaf oil bodies function in stress responses.…”