Recognition signals within a target protein can be:

1. The N-terminal amino acid (N-end rule)
   stabilizing aa:      Cys, Ala, Ser, Thr, Gly, Val, Met
   short-lived proteins:  Arg, Lys, Phe, Leu Trp
    
2. Secondary modifications
   phosphorylations
   hydroxylations
    
3. Special amino acid sequences (destruction boxes or degrons), i.e. PEST regions = amino acid sequences rich in proline (P), glutamate (E), serine (S) and threonine (T).

References

Degradation of proteins by the ubiquitin-proteasome-system