Abstract: Proteins and fusion proteins for forming merbraneless droplets in cells are provided. Described herein, is the development of a protein, named PopTag, that drives phase separation when it is part of a chimeric fusion protein. PopTag is engineered from the PopZ protein, found in a-proteobacteria (including Caulobacter crescentus). Despite PopZ being exclusively found in this clade of bacteria, the PopTag can drive protein phase separation in other prokaryotes (e.g., E. coli) and eukaryotes (e.g., human cells).