Structural and Evolutionary Analysis of Non-specific Lipid Transfer Proteins (nsLTPs) in Cereus (Cactaceae).

The genus Cereus (Cactaceae), widely distributed across arid and semi-arid environments of South America, exhibits adaptive capacity that is partly associated with proteins involved in lipid metabolism and stress responses, such as non-specific lipid transfer proteins (nsLTPs). Although well-characterized in model plants, the structural diversity and functional roles of these proteins in cacti remain largely unexplored. In this study, we characterized the nsLTP repertoire of Cereus by integrating transcriptomic data, phylogenetic analyses, structural modeling, and ligand-binding site mapping. Structural comparisons revealed conservation of the typical nsLTP fold, with variations mainly localized to loop regions surrounding the binding cavity, suggesting functional plasticity without compromising scaffold stability. Phylogenetic analyses revealed frequent duplication and loss events, as well as species-specific paralogs. Several lineage-specific copies exhibited distinctive loop architectures and pocket geometries relative to their closest orthologs, suggesting rapid structural diversification following duplication. This pattern is consistent with functional fine-tuning after gene duplication. Positive selection analyses identified nine codons with significant evidence of episodic diversifying selection (p < 0.05), some of which mapped near predicted functional regions, supporting the hypothesis that selective pressures have shaped key interaction interfaces. Together, these results provide the first integrated characterization of nsLTPs in Cereus, supporting a gate-modulation hypothesis in which structural variation concentrated at tunnel entrances may contribute to functional diversification while preserving the conserved nsLTP fold (four-helix scaffold). Rather than asserting adaptation directly, our findings provide a structural basis and testable predictions for how nsLTP diversification may influence lipid trafficking processes relevant to cuticle/membrane function under abiotic stress.