Abstract: The preset disclosure provides chimeric polypeptides comprising a cage polypeptide comprising a degron, wherein the degron is sequestered or caged. Upon activation by a key polypeptide, the degron becomes active. The degron can recruit an ubiquitin ligase and a lysine in the degron or surrounding sequence be ubiquitinated. The ubiquitinated chimeric polypeptide is marked for degradation, together with any biologically active molecule attached to the chimeric polypeptide. The chimeric polypeptides of the present disclosure can be incorporated, for example, to chimeric antigen receptors (CAR). Accordingly, in response to the administration of a key polypeptide or endogenous expression of a key polypeptide mediated, for example, by an inducible promoter, the amount of CAR expressed on the surface of an immune cell can be modulated.