How Lipid Droplets Select and Grow

Lipid droplets store lipids, but their total amount alone does not explain their state or function. Recent studies examined how lipids and proteins are delivered to droplets, where proteins localize, and how delivered lipids support growth. One study showed that ER-derived membrane proteins become selectively enriched on droplets through transient capture at specific surface regions. The other showed that ER-delivered lipids not only serve as storage material but also recruit the converting enzyme to the droplet surface, driving growth of existing droplets. These findings indicate that lipid droplets are dynamic structures that grow by accepting lipids and proteins and recruiting enzymes, rather than simple storage sites. Assessing their state requires evaluating number, size, lipid incorporation, and protein localization separately.

Membrane bridges and nanodomain partitioning govern membrane protein targeting to lipid droplets
(Mizrak et al., Nature Cell Biology, 2026)

Summary: 
In this study, membrane proteins that function on lipid droplets were tracked one molecule at a time in human breast cancer-derived SUM159 cells. The proteins moved in both directions through membrane connections between the endoplasmic reticulum and lipid droplets. After reaching a lipid droplet, they were temporarily captured in small regions on its surface. This clarified that proteins accumulate not simply by moving onto lipid droplets, but by being selectively retained in specific surface regions.

Highlighted technique: 
To examine protein movement and surface localization, fluorescently labeled lipid-droplet proteins were tracked one molecule at a time in living SUM159 cells. Lipid droplets were stained separately, and the release of a model protein from the endoplasmic reticulum was synchronized. High-resolution microscopy and computer-based analysis mapped movement routes and local accumulation.

The four-color Lipi series enables selection according to the fluorescent labels used and the imaging goal. All four provide clear, low-background staining of lipid droplets in live and PFA-fixed cells. For example, blue retains fluorescence for 24 hours and shows high spatial agreement with immunofluorescence staining, and deep red pairs well with blue or green labels. 

ATG2A-mediated DAG transfer recruits DGAT2 for lipid droplet growth 
(Elhan et al., Nature Structural & Molecular Biology, 2025)

Summary: 
In this study, HeLa cells lacking two related lipid-transfer proteins were used to examine how newly supplied lipids are stored in lipid droplets. Although total droplet volume remained similar, less lipid entered existing droplets and many smaller droplets formed. The lipid delivered to droplets also attracted an enzyme that converted it into a storage form, promoting the growth of existing droplets. This clarified why both enlargement of existing droplets and formation of new droplets should be evaluated.

Highlighted technique: 
To evaluate how lipid delivery supports the growth of existing droplets, HeLa cells were first given oleic acid to form mature lipid droplets and then exposed to fluorescent fatty acids and a lipid-droplet dye. Live confocal microscopy was used to assess lipid incorporation into existing droplets, droplet number and size, and enzyme localization.

Lipid droplets can be evaluated quantitatively and morphologically by combining flow cytometry and imaging. Flow cytometry measures cellular droplet content and population variation, while imaging reveals droplet number, size, and shape..