Caveosomes and Endocytosis of Lipid Rafts

Endocytosis of various endogenous plasma membrane molecules, including signalling receptors, glycosphingolipids and glycosylphosphatidylinositol (GPI)-linked proteins, occurs in the absence of functional clathrin-coated pits. Most of these molecules are found in biochemically defined lipid rafts, which suggests that at least some clathrin-independent endocytosis may be raft specific or raft mediated. However, recent studies of the uptake of raft markers have revealed a diversity of internalization methods. Although lipid rafts may somehow be recognized by endocytic machinery, at this stage the data do not readily fit with the idea of a single raft-specific or raft-dependent endocytic pathway. Many studies report uptake of raft molecules by caveolar endocytosis (defined by sensitivity to cholesterol depletion and to overexpression of a specific mutant of dynamin 2). It is now apparent that this is a highly regulated process, and caveolin 1, one of the characteristic protein components of caveolae, might in fact act to slow or inhibit endocytosis. The molecular details of caveolar endocytosis have yet to be elucidated. Several sources indicate that clathrin-independent uptake to a distinct class of caveolin-1-containing endosome, termed the caveosome, allows different types of endocytic mechanisms to have different functional consequences for the cell. It is likely that there are mechanisms that allow recruitment and targeting of specific molecules to caveosomes.