Nanocondensate bioadhesive delivery via metal-halogenated catechol coordination in tunicate rhizoid holdfasts

The root-like holdfast of the tunicate Halocynthia roretzi provides strong underwater adhesion. However, the biological processing and biochemical composition underlying its adhesive remain largely unknown. Here, we identify a nanocondensate-based transport system in which halogenated 3,4-dihydroxyphenylalanine (DOPA)-containing peptides coordinate with metal ions such as iron, chromium, and vanadium to form stable nanocondensates within dense-granular cells. These nanocondensates are secreted into the extracellular matrix and rapidly incorporated into the cuticular layer, where the proteins cross-link oxidatively to form the adhesive interface, releasing the metals upon solidification. This process establishes a previously unrecognized solid-state adhesive delivery mechanism regulated by coordination chemistry between metal ions and halogenated catechols. Indeed, while other systems (e.g., mussels) use DOPA-containing proteins to transport metal ions during glue formation, the current system is distinctive in that metal coordination is transient and used ostensibly to deliver the adhesive protein cargo-findings relevant for design of next-generation underwater glues.