The "signals" or amino acid "motifs" in the cargo proteins that interact with the adaptor proteins can be very short. For example, one well-known example is the dileucine motif, in which a leucine amino acid (aa) residue is followed immediately by another leucine or isoleucine residue. An even simpler example is the tyrosine based signal, which is YxxØ (a tyrosine residue separated by 2 aa residues from another bulky, hydrophobic aa residue). The accompanying figure shows how a small part of a protein can interact specifically with another protein, so these short signalling motifs should not be surprising. The sort of sequence comparisons used, in part, to define these motifs.