Clathrin-Independent Pathways of Endocytosis

There are many pathways of endocytosis at the cell surface that apparently operate at the same time.With the advent of newmolecular genetic and imaging tools, an understanding of the differentways bywhich acellmayendocytose cargo is increasing by leaps and bounds. In this review we explore pathways of endocytosis that occur in the absence of clathrin. These are referred to as clathrin-independent endocytosis (CIE). Here we primarily focus on those pathways that function at the small scale in which some have distinct coats (caveolae) and others function in the absence of specific coated intermediates. We follow the trafficking itineraries of the material endocytosed by these pathways and finally discuss the functional roles that these pathways play in cell and tissue physiology. It is likely that these pathwayswill play key roles in the regulation of plasma membrane area and tension and also control the availability of membrane during cell migration. The identification ofmanyof the componentsinvolved in clathrin-mediated endocytosis (CME) and their subsequent characterization have provided a window into how this complex process works. For example, understanding how a clathrin basket is assembled, and how adaptor complexes, the mechanochemical GTPase dy-namin, and Rab GTPases work have given us insights into endocytic pit formation, cargo concentration, vesicle scission, and subsequent trafficking. These topics are described in detail elsewhere in this volume (see Johannes et al