Study on Phosphatidylethanolamine-binding Protein Required for Degradation of Lipid Droplets in Chlamydomonas reinhardtii

Abstract

Lipid droplets (LD) are intracellular organelles found in a wide range of organisms, playing important roles in stress tolerance. During nitrogen (N) starvation, Chlamydomonas reinhardtii stores large amounts of triacylglycerols (TAG) inside LDs. When N is resupplied, the LDs disappear and the TAGs are degraded, presumably providing carbon and energy for re-growth. The mechanism by which cells degrade LDs is poorly understood. Here, I isolated a mutant (crdth1-1, Delayed in TAG Hydrolysis 1) in which TAG degradation during recovery from N starvation was compromised. Consequently, the crdth1-1 mutant grew poorly compared to its parental line during N recovery. Two additional independent loss-of-function mutants (crdth1-2 and crdth1-3) also exhibited delayed TAG remobilization. CrDTH1 transcript levels increased seven-fold upon N resupply, and CrDTH1 protein was localized to LDs. CrDTH1 contains a putative lipid-binding domain (DTH1LBD) with alpha helices predicted to be structurally similar to those in apolipoproteins E and A-I. Recombinant DTH1LBD bound specifically to phosphatidylethanolamine (PE), a major phospholipid coating the LD surface. Overexpression of DTH1LBD in Chlamydomonas phenocopied the dth1 mutant’s defective TAG degradation, suggesting that the function of DTH1 depends on its ability to bind PE. Together, these results demonstrate that CrDTH1 plays an essential role in LD degradation and provide an insight on the molecular mechanism of protein anchorage to LDs at the LD surface in photosynthetic cells.