The present invention relates to a polypeptide multimer comprising a first polypeptide having associated therewith a label and a second polypeptide, wherein a) at least one of the polypeptides is susceptible to protease digestion; b) association of the polypeptides to form a multimer is detectable via a signal emitted by the label; and c) digestion of at least one polypeptide results in dissociation of the multimer and modulation of the signal emitted by the label, and the method of detecting or monitoring the activity of a protease enzyme based on such a multimer.