Elucidation of Clathrin-Mediated Endocytosis in Tetrahymena Reveals an Evolutionarily Convergent Recruitment of Dynamin

Abstract

Ciliates, although single-celled organisms, contain numerous subcellular structures and pathways usually associated with metazoans. How this cell biological complexity relates to the evolution of molecular elements is unclear, because features in these cells have been defined mainly at the morphological level. Among these ciliate features are structures resembling clathrin-coated, endocytic pits associated with plasma membrane invaginations called parasomal sacs. The combination of genome-wide sequencing in Tetrahymena thermophila with tools for gene expression and replacement has allowed us to examine this pathway in detail. Here we demonstrate that parasomal sacs are sites of clathrin-dependent endocytosis and that AP-2 localizes to these sites. Unexpectedly, endocytosis in Tetrahymena also involves a protein in the dynamin family, Drp1p (Dynamin-related protein 1). While phylogenetic analysis of AP subunits indicates a primitive origin for clathrin-mediated endocytosis, similar analysis of dynamin-related proteins suggests, strikingly, that the recruitment of dynamin-family proteins to the endocytic pathway occurred independently during the course of the ciliate and metazoan radiations. Consistent with this, our functional analysis suggests that the precise roles of dynamins in endocytosis, as well as the mechanisms of targeting, differ in metazoans and ciliates. The wings of bats and of birds are similar structures with similar functions but nonetheless evolved independently within these two different branches of animals. Many examples of this phenomenon, called convergent evolution, are known at the level of whole organisms. Here, the authors demonstrate that convergent evolution has also occurred at the level of individual cells, in a pathway responsible for taking up membrane from the cell surface. The authors took advantage of the recent genomic sequencing of distantly related organisms, and in particular of the single-celled ciliate Tetrahymena thermophila. In animal cells, one of the proteins required for membrane uptake is called dynamin. Dynamin is not required for this function in most nonanimal cells, but the authors discovered that Tetrahymena is an exception and that it uses a close relative of dynamin for particle uptake. After reconstructing the history of dynamin proteins, the authors found that the specific role in membrane uptake evolved independently in Tetrahymena and in animals.