Proteomics analysis of lipid droplets indicates involvement of membrane trafficking proteins in lipid droplet breakdown in the oleaginous diatom Fistulifera solaris

Microalgae store triacylglycerol (TAG) in a specialized intracellular compartment called the lipid droplet. Its dynamics are believed to be controlled by various lipid droplet-associated proteins, although studies of microalgal lipid droplet-associated proteins are still limited. In the present study, proteomics analysis was carried out to identify novel lipid droplet-associated protein candidates in the oleaginous marine diatom Fistulifera solaris JPCC DA0580. Lipid droplets were extracted from diatom cells, followed by washing with urea solution to decrease contaminants. Subsequently, proteins were identified from the urea-treated lipid droplet fraction. Comparisons between the present study and previous proteomic research on other microalgae revealed that the lipid droplet fraction extracted from diverse microalgae commonly contained membrane trafficking-related proteins such as vesicle coat proteins and heat shock proteins, some of which could potentially be involved in autophagy. Autophagy is known to degrade lipid droplets in other organisms (i.e. mammalian cells and yeasts), and this process is termed lipophagy. Thus, the membrane trafficking-related proteins which were detected from the proteome in this study might play a role in TAG hydrolysis via lipophagy. In order to confirm this hypothesis, we utilized inhibitors of membrane trafficking-related proteins and lipophagy. The addition of the inhibitors to F. solaris during lipid droplet breakdown suppressed TAG hydrolysis, supporting the idea that lipophagy may participate in TAG hydrolysis in F. solaris. The phenomenon was also observed in another model pennate diatom, Phaeodactylum tricornutum, implying that lipophagy-mediated lipid droplet breakdown could be a common feature, at least in pennate diatoms. These findings shed light on novel aspects of lipid metabolism in microalgae, and this can be applied to new approaches for increasing the oil productivity of biofuel production using microalgae in the future.