Serra, N. D.; Chen, J.; Birnbaum, S. K.; Aviles, S. G.; Sundaram, M. V.

Apical extracellular matrices (aECMs) line exposed body surfaces to shape tissues and protect them from the environment. These aECMs often organize into complex patterns and structures, but how such matrices assemble remains poorly understood. Caenorhabditis elegans cuticle patterns initiate within the transient pre-cuticle, which then helps direct the placement of cuticle collagens. Pre-cuticle patterns arise through post-secretory sorting, which must involve specific molecular interactions among them. Consistent with such a model, Alphafold3 predicts a high confidence physical interaction between two pre-cuticle proteins, the lipocalin LPR-3 and the Hedgehog-related protein WRT-10, with a conserved N-terminal region of LPR-3 forming a beta-strand that incorporates into the beta-barrel-like structure of the WRT-10 WRT domain. Genetic studies showed that WRT-10 requires this LPR-3 region in order to become properly patterned in the pre-cuticle matrix. Furthermore, WRT-10 and the LPR-3 beta-strand region are required to pattern a specific cuticle substructure, the lateral alae ridges, but not for other LPR-3-dependent matrix roles. These data indicate that LPR-3 and WRT-10 are functional partners and support a "landing pad" model whereby physical interactions between them allow LPR-3 to recruit WRT-10 to specific aECM regions. Similar mechanisms may explain how other members of the C. elegans Hh-r family associate with the aECM.