“pioneering the Xenopus Oocyte and Egg System”

A “Reflections” article provides an opportunity to focus on key findings during a career from a personal and historical perspective. As a graduate student in the late 1960’s at Berkeley, I was introduced to the Xenopus oocyte by John Gerhart, who shared his fascination with the work of John Gurdon that had pointed out the unique and powerful features of the oocyte and egg,, particularly the ability to microinject biochemically significant amounts of material into them while they carried out complex biological events. This approach using the oocyte as a living biochemical test tube long predated the age of cloning and gene transfection. Later, the synchronous, hormone-stimulated transition of the oocyte from G2 to M phase during maturation encouraged a focus on cell cycle control. Key events over the years include identifying changes in protein phosphorylation as the initiating signal for oocyte maturation and more universally for the action of maturation-promoting factor (MPF) to drive cells into M phase. A crucial advance came from developing methods to produce large quantities of concentrated egg extracts that could carry out MPF-dependent cell cycle transitions, DNA replication, and other complex processes in vitro. M phase phosphorylation analysis led to purification of p90Rsk and discovery of the MAPK pathway, which underlies many biological events, including the metaphase arrest of the unfertilized egg. A seminal breakthrough came from purifying MPF from egg extracts and showing that it was a complex of the Cdc2 kinase and cyclin B, uniting the genetic and biochemical approaches to cell cycle control. The oocyte and egg extract system continues to provide new insight into many areas of biology, including the action of other mitotic kinases and centrosome duplication. The unique and powerful features of the Xenopus system hold great promise for more work and insight into fundamental problems of biology and medicine.