The phosphatidylethanolamine-binding protein DTH1 mediates degradation of lipid droplets in <i>Chlamydomonas reinhardtii</i>

Abstract

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