Factors Limiting Expression of Secreted Proteins in Mammalian Cells

Gene expression may be controlled at the level of transcription, processing of precursor mRNAs, mRNA transport to the cytoplasm, mRNA stability and translational efficiency, and protein stability. Proteins which transit the secretory apparatus are subject to a variety of additional steps which may regulate the appropriate secretion of the mature polypeptide. Experience from studying the expression of a wide variety of proteins which transit the secretory apparatus demonstrates that the rate limiting step in secretion is transport from the endoplasmic reticulum (ER) to the Golgi apparatus. It is now known that expression of heterologous secretory proteins from transfected genes introduced into heterologous cells may also be limited due to inefficient transport from the ER to Golgi. In addition, a wide variety of post-translational steps which are required for appropriate maturation and biological activity of a protein may be saturated as the expression of the specific protein is increased. Our work has focused on the limiting steps for the secretion of these heterologous proteins expressed at high level in Chinese hamster ovary (CHO) cells. In general, the factors limiting expression of these proteins are at the post-translational events which occur within the secretory pathway.