Provided herein are polypeptide constructs containing a securin, e.g., a full-length securin or a truncated securin, fused to a separase. The polypeptide constructs may further contain linker peptides, protease recognition sites, and unfoldase recognition sites to facilitate expression and/or purification. Methods for obtaining the polypeptide constructs with active separase activity are also described, as well as methods for identify separase modulator compounds such as separase inhibitors.

Problem to be Solved An object of the present invention is to provide a separase sensor for visualizing separase activity in a living cell. Another object is to develop a function analysis system of cell division using the separase sensor and further obtain a simple screening method for an anticancer agent. Solution The separase sensor of the present invention has two types of fluorescent proteins different in fluorescence wavelength at both ends of an amino acid sequence containing a separase cleavage site and a localization-targeting sequence targeting to a specific site within a cell. Since timing of activating separase and location of separase in a cell can be specified by use of the sensor, cell division can be analyzed and an anticancer agent can be screened.