Secretion of proteins by the fertilized mouse ovum

Fertilized one-cell mouse ova were injected with messenger RNA (mRNA) for chicken ovalbumin or rat albumin. The ova were labeled with ³H]amino acids, and endogenous proteins as well as proteins in the culture medium were separated by two-dimensional electrophoresis. Following injection of either message, ova were able to synthesize and secrete the appropriate protein. However, when the message for globin was injected, globin appeared only among the endogenous ovum proteins but was not secreted into the culture medium. The location of the secreted proteins, albumin and ovalbumin, on two-dimensional gels suggests that ova process the proteins before secretion by removing the amino-terminal signal sequence from rat albumin and glycosylating ovalbumin. Rat albumin may be exported into the culture medium more rapidly than ovalbumin. In addition, at least one endogenous ovum protein was secreted at the one-cell stage. The studies establish the ability of the newly fertilized mouse ovum to synthesize and secrete proteins from injected as well as endogenous messages.