A protein construct including a gene encoding a light-sensitive protein fused to at least one of either a low complexity sequence, an intrinsically disordered protein region (IDR), or a repeating sequence of a linker and another gene encoding a light-sensitive protein. Among the many different possibilities contemplated, the protein construct may also advantageously include cleavage tags. This protein construct may be utilized for a variety of functions, including a method for protein purification, which requires introducing the protein construct into a living cell, and inducing the formation of clusters by irradiating the construct with light. The method may also advantageously include cleaving a target protein from an IDR, and separating the clusters via centrifuge. A kit for practicing in vivo aggregation or liquid-liquid phase separation is also included, the kit including the protein construct and a light source capable of producing a wavelength that the light-sensitive protein will respond to.