The Oocyte as a Secretory Cell

Xenopus laevis oocytes secrete a large variety of foreign secretory proteins after the microinjection of mRNA or DNA. Two classes of such proteins are discussed in detail. These are the chick oviduct proteins ovalbumin and lysozyme, and the mouse MOPC 21 immunoglobulin. The injection of mRNAs for mouse immunoglobulin heavy or light chain leads to the synthesis, segregation, but not secretion of the encoded proteins unless the two mRNAs are simultaneously or sequentially injected into the same oocytes. Chicken ovalbumin and lysozyme are synthesized and secreted from oocyte after the injection of either oviduct mRNA or cloned DNA (ovalbumin). The secreted lysozyme is exported considerably faster than ovalbumin; however, 40% of the lysozyme synthesized cannot be secreted and, after fractionation of oocytes on sucrose gradients, is found in a higher density position than ovalbumin. No competition at the level of secretion or translation was noted when different amounts of immunoglobulin and ovalbumin mRNAs were injected into oocytes. However, the co-injection of ovalbumin mRNA and mRNAs encoding anti-ovalbumin immunoglobins resulted in the formation of a complex of the two types of protein within the oocyte. In these circumstances, secretion of the immunoglobulin was severely reduced.