The c-mos proto-oncogene protein kinase turns on and maintains the activity of MAP kinase, but not MPF, in cell-free extracts of Xenopus oocytes and eggs.

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Abstract
During studies of the activation and inactivation of the cyclin B‐p34cdc2 protein kinase (MPF) in cell‐free extracts of Xenopus oocytes and eggs, we found that a bacterially expressed fusion protein between the Escherichia coli maltose‐binding protein and the Xenopus c‐mos protein kinase (malE‐mos) activated a 42 kDa MAP kinase. The activation of MAP kinase on addition of malE‐mos was consistent, whereas the activation of MPF was variable and failed to occur in some oocyte extracts in which cyclin A or okadaic acid activated both MPF and MAP kinase. In cases when MPF activation was transient, MAP kinase activity declined after MPF activity was lost, and MAP kinase, but not MPF, could be maintained at a high level by the presence of malE‐mos. When intact oocytes were treated with progesterone, however, the activation of MPF and MAP kinase occurred simultaneously, in contrast to the behaviour of extracts. These observations suggest that one role of c‐mos may be to maintain high MAP kinase activity in meiosis. They also imply that the activation of MPF and MAP kinase in vivo are synchronous events that normally rely on an agent that has still to be identified.