One of three means for a protein molecule to pass between compartments within eucaryotic cells. The compartment “wall” (membrane) possesses a “sensor” (receptor) that detects the presence of correct protein (e.g., after that protein has been synthesized in the cell’s ribosomes), then bulges outward along with that protein molecule. The membrane bulge-containing protein then “breaks off” and carries (transports) the protein to its destination in another compartment in the cell.