It has been considered that the promotion of the formation of a polysome, which is a substance composed of mRNA and many ribosomes attached to the mRNA, is very effective for the highly efficient synthesis of a protein. However, the mechanism of the polysome formation promotion function of p180 protein was not revealed yet. The inventors of the present application newly discovered SF3b4 protein as a protein that can interact specifically with a coiled-coil domain, which is a domain responsible for the polysome formation promotion function of p180 protein, and as a protein that can promote the localization of mRNA in an endoplasmic reticulum (ER), and the inventors found that the localization of mRNA in an endoplasmic reticulum is significantly increased and the secreting function of a cultured cell can be enhanced in a cell that can express both a protein capable of promoting the localization of mRNA in an endoplasmic reticulum (ER) (e.g., SF3b4 protein) and p180 protein at high levels. It is demonstrated that SF3b4 protein, which is a protein having a protein expression enhancement effect, can be localized on the membrane of an endoplasmic reticulum and the weight of mRNA in a polysome can be shifted to a high level by inserting a specific nucleotide sequence into an expression plasmid, and it is also demonstrated that the secreting ability of a cell can be increased through the shift.
