The function of the UBX-domain containing scaffold protein FAF1/PUX10 in lipid droplet degradation in seedlings and pollen tubes
Plant Biochemistry and Biophysics
Final Report Abstract
Lipid droplets (LDs, also known as oleosomes or oil bodies in plants) are ubiquitous ER-derived subcellular compartments consisting of a hydrophobic matrix surrounded by a phospholipid monolayer and proteins. The hydrophobic matrix is dominated in most cases by triacylglycerols (TAGs)that are an energy- and carbon-dense class of lipids important for example during seed germination. While the lipase that degrades most of the TAG during seed germination is well known, it is less clear how the proteins associated with LDs are degraded. We had previously identified a LD-associated scaffold protein PUX10 (for plant UBX domain containing-protein) that we suspected to play a role in protein degradation. We could now show that this protein can recruit CDC48a to lipid droplets via its UBX domain. CDC48 is a protein that can unfold other proteins by the expense of ATP, so that these can be degraded by the proteasome. By stably expressing a fluorophore-tagged version of PUX10 under the native promoter in Arabidopsis, we could show that it is expressed in pollen tubes, developing embryos (starting from the heart stage) and seedlings. It is in all stages localized at LDs, but in seedlings especially to a subpopulation of LDs. Our studies further revealed that PUX10 is involved in the degradation of LD proteins during seed germination as this degradation is slowed in a pux10-1 knockout mutant. This effect could be partially complemented. In another knockout mutant, pux10-3, in the Wassilevskia ecotype, the level of LD proteins was already increased in the seeds speaking for a decreased turnover during seed maturation. On the other hand the pux10-1 mutant was not affected in germination at least under optimal condition. TAG degradation and seedling establishment were not hampered and also pollen tube growth was not affected. In addition to our work on PUX10, we could identify from our proteomic datasets several proteins that were strongly enriched in LD fractions. Six of these proteins, all from different families, we could demonstrate to indeed target to LDs when transiently expressed coupled to fluorophores, thereby substantially increasing the number of known LD protein families from plants (less than 20 are known from vascular plants as of now). One of these proteins of previously unknown function can interact with another protein of previously unknown function to target LDs to the plasma membrane.
Publications
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(2017) Arabidopsis lipid drop-associated protein (LDAP) - interacting protein (LDIP) influences lipid droplet size and neutral lipid homeostasis in both leaves and seeds. The Plant Journal, 92, 1182-1201
Pyc, M., Cai, Y., Gidda, S.K., Yurchenko, O., Park, S., Kretzschmar, F.K., Ischebeck, T., Valerius, O., Braus, G.H., Chapman, K.D., Dyer, J.M. and Mullen, R.T.
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(2018) PUX10 is a lipid droplet-localized scaffold protein that interacts with CDC48 and is involved in the degradation of lipid droplet proteins. The Plant Cell. 30, 2137-2160
Kretzschmar, F.K., Mengel, L.F., Müller, A., Schmitt, K., Blersch, K.F., Valerius, O., Braus, G., and Ischebeck, T.
