Abstract: The invention relates to a modular polypeptide comprising a first partial effector sequence comprising a first part of a circular permutated halotag protein connected to a sensor module sequence, which is connected to a second part of a circular permutated halotag protein. The sensor module is a single polypeptide or a polypeptide pair capable of undergoing conformational change from a first confirmation to a second confirmation depending on the presence or concentration of an analyte compound. The modular peptide is catalytically active in response to an environmental stimulus or in response to the sensor pair interacting. The invention further relates to nucleic acid sequences encoding the modular polypeptide, and to kits comprising same.