 As examples, membrane proteins can have direct interactions, while proteins that are soluble in the lumen of the donor organelle bind indirectly to AP complexes by binding to membrane proteins that traverse the membrane and bind at their lumenal end to the desired cargo molecule.Molecules that should not be included in the vesicle appear to be excluded by "molecular crowding".The "signals" or amino acid "motifs" in the cargo proteins that interact with the adaptor proteins can be very short.