An Sfp2-like protein underlies tube foot temporary adhesion in the sea cucumber Apostichopus japonicus

Temporary adhesion by echinoderm tube feet is essential for attachment, locomotion and feeding, yet its molecular basis remains poorly understood in Holothuroidea. Here, we combined ultrastructural, proteomic, phylogenetic and expression analyses to investigate the temporary adhesive system in the sea cucumber Apostichopus japonicus. Scanning and transmission electron microscopy revealed a highly specialized tube foot disc epidermis featuring abundant secretory pores, dense microvilli, and two distinct types of adhesive secretory cells, whereas these features were absent in the non-adhesive homologous papillae. Proteomic analysis of tube foot footprints and discs, combined with bioinformatic screening, identified 20 high-confidence candidate adhesive proteins and one Astacin-domain-containing candidate de-adhesive protein. Notably, the 4,085-aa multidomain protein Aj-Sfp2-like was relatively abundant in the footprint proteome and showed extensive predicted post-translational modification. Tissue expression and in situ localization confirmed that Aj-Sfp2-like transcripts are localized to the secretory epidermis of adhesive appendages (tube feet and tentacles), whereas no such expression was detected in the non-adhesive papillae. Phylogenetic and comparative domain analyses demonstrate that Aj-Sfp2-like belongs to the broadly conserved Sfp2-like protein family, sharing a homologous modular framework with asteroid-restricted Sfp1-like proteins. These findings provide the first molecular framework for understanding tube foot adhesion and de-adhesion in holothurians and broaden our understanding of the conserved and divergent basis of temporary adhesion across echinoderms.