Arabidopsis thaliana Fibrillin 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.