Abstract
In animals, cell-matrix adhesions are essential for cell migration, tissue organization, and differentiation, which have central roles in embryonic development [1-6]. Integrins are the major cell surface adhesion receptors mediating cell-matrix adhesion in animals. They are heterodimeric transmembrane proteins that bind extracellular matrix (ECM) molecules on one side and connect to the actin cytoskeleton on the other [7]. Given the importance of integrin-mediated cell-matrix adhesion in development of multicellular animals, it is of interest to discover when and how this machinery arose during evolution. Comparative genomic analyses have shown that core components of the integrin adhesome pre-date the emergence of animals [8-11]; however, whether it mediates cell adhesion in non-metazoan taxa remains unknown. Here, we investigate cell-substrate adhesion in Capsaspora owczarzaki, the closest unicellular relative of animals with the most complete integrin adhesome [11, 12]. Previous work described that the life cycle of C. owczarzaki (hereafter, Capsaspora) includes three distinct life stages: adherent; cystic; and aggregative [13]. Using an adhesion assay, we show that, during the adherent life stage, C. owczarzaki adheres to surfaces using actin-dependent filopodia. We show that integrin β2 and its associated protein vinculin localize as distinct patches in the filopodia. We also demonstrate that substrate adhesion and integrin localization are enhanced by mammalian fibronectin. Finally, using a specific antibody for integrin β2, we inhibited cell adhesion to a fibronectin-coated surface. Our results suggest that adhesion to the substrate in C. owczarzaki is mediated by integrins. We thus propose that integrin-mediated adhesion pre-dates the emergence of animals.