Final Report Summary - BFLDS (Direct Imaging of Budding and Fusion of Lipid Droplets Mediated by Proteins in Emulsion Droplets Based on Microfluidics - Dynamics of Proteins Interactions, Assembly and Metabolism Energy)
Lipid droplets (LDs) are cellular fat droplets at the core of cellular energy metabolism. Their regulation is crucial for human health. A malfunction of LDs has direct consequences such as the development of cardiovascular diseases and type II diabetes, and many other lipid pathologies including liver steatosis or lipodystrophy. LDs perform many other functions, distinct from the basic regulation or cellular energy metabolism, and serve as hosts for the proliferation of Hepatitis C and Dengue viruses. The fate of LDs and their consequences on human health is based on the specific binding of proteins to their surface. During the three years term of the BFLD project, I tackled important questions of how proteins localize to lipid droplets (LDs) and how is their binding regulated. This is the main question of the LD field.
1) I contributed to unveil the role the vesicular trafficking machinery COPI on controlling LD proteins targeting to LDs. We showed that COPI increases the surface tension of LDs to favor either the direct binding of proteins to LDs, as their triglyceride oil phase become exposed, or the establishment of bridges between the ER and LDs to enable the passage of proteins.
2) We have demonstrated that for such process, COPI provides and energy of 1500-2000 kBT to bud nano particles from membranes, especially from LD monolayers. Knowing this energy allows better understanding how COPI budding is mechanically regulated in vivo by a simple remodeling of membrane properties.
3) The binding of proteins to LDs determines LD protein composition and fate. We demonstrated that proteins compete for binding LD. Proteins have different binding motifs. We found that amphipathic helices such as of CCT1, an important enzyme of phosphatidyl choline synthesis, are competed off from LDs by other strongly binding proteins, e.g. GPAT4 having a hairpin-binding motif. This competition for binding is best illustrated by lipolysis during energy remobilization. Indeed, LDs get consumed, shrink and diminish their surface. The compression of the LD surface during this process leads mainly to expel of amphipathic helix-binding proteins.
1) I contributed to unveil the role the vesicular trafficking machinery COPI on controlling LD proteins targeting to LDs. We showed that COPI increases the surface tension of LDs to favor either the direct binding of proteins to LDs, as their triglyceride oil phase become exposed, or the establishment of bridges between the ER and LDs to enable the passage of proteins.
2) We have demonstrated that for such process, COPI provides and energy of 1500-2000 kBT to bud nano particles from membranes, especially from LD monolayers. Knowing this energy allows better understanding how COPI budding is mechanically regulated in vivo by a simple remodeling of membrane properties.
3) The binding of proteins to LDs determines LD protein composition and fate. We demonstrated that proteins compete for binding LD. Proteins have different binding motifs. We found that amphipathic helices such as of CCT1, an important enzyme of phosphatidyl choline synthesis, are competed off from LDs by other strongly binding proteins, e.g. GPAT4 having a hairpin-binding motif. This competition for binding is best illustrated by lipolysis during energy remobilization. Indeed, LDs get consumed, shrink and diminish their surface. The compression of the LD surface during this process leads mainly to expel of amphipathic helix-binding proteins.
