A PEST sequence is a peptide sequence that is rich in proline (P), glutamic acid (E), serine (S), and threonine (T). This sequence is associated with proteins that have a short intracellular half-life; hence, it is hypothesized that the PEST sequence acts as a signal peptide for protein degradation.
The degradation may be mediated possibly via the proteasome or calpain.
References
PEST sequence Wikipedia(Text) CC BY-SA