Endocytosis, the sorting problem and cell locomotion in fibroblasts.

Fibroblasts endocytose lipid plus a subset of plasma membrane proteins over their entire surface and reinsert this into the plasma membrane at the cell's leading edge. This process is used to extend the fibroblast forwards. This circulation causes a flow of these endocytosed molecules over the cell's surface. Molecules, such as proteins, sitting in this flow can distribute themselves randomly by Brownian motion, but large objects (or small tethered ones) cannot. These large objects therefore cap. A mechanism is presented whereby this process could be used for locomotion using many weak interactions with the substrate. In addition it is suggested that the observed selectivity of coated pits may be sufficient to sort out proteins during transfer of membrane from one organelle to another so that the specific characters of the parent membranes are maintained.