What is the purpose of using fusion tags for expression?

Fusion tags are used for detecting and purifying the target protein. Sometimes they are also used to increase the solubility of the target protein in the cytoplasm or to assist in transporting the target protein to the periplasm to enhance its biological activity. Common soluble tags like GST, MBP, and SUMO are often added to prokaryotic expression vectors or protein sequences. These tags are highly soluble when expressed in the host. When fused to a foreign protein, they can promote the soluble expression of the foreign protein.