Arabidopsis thalianaFibrillin 3a binds saturated fatty acids of chloroplast membranes

Fibrillins (FBNs) are conserved plastid lipid–associated proteins involved in lipid storage, stress adaptation and plastid ultrastructure. While several Arabidopsis thaliana FBNs have been functionally characterized, the biochemical properties of the thylakoid-associated FBN3a remain poorly understood. AlphaFold modeling revealed that AtFBN3a adopts an eight-stranded β-barrel fold typical of the lipocalin family, with high-confidence predictions for the core β-strands and conservation of the structurally conserved region 1 (SCR1) motif that stabilizes the barrel and defines the ligand-binding cavity. This structural topology is also shared with AlphaFold models of the other members of the family in A. thaliana. Consistent with the lipocalin-like structural and sequence features, protein–lipid overlay assays showed that AtFBN3a bound the anionic plastid lipids phosphatidic acid and sulfoquinovosyl diacylglycerol, but not phosphatidylcholine. Additional assays revealed a clear preference for saturated fatty acids, with stronger binding to long-chain saturated species. Together, these findings identify AtFBN3a as a lipocalin-like domain–containing protein with selective affinity for saturated fatty acids, suggesting a conserved role for FBNs in plastid lipid metabolism and stress adaptation.