Identification and comparative analysis of multiple mammalian Speedy/Ringo proteins

In addition to their activation via binding to cyclins, cyclin-dependent kinases (CDKs) can be activated via binding to a novel cell cycle regulator termed Speedy/Ringo, which shows no apparent similarity to cyclins. The first Speedy/Ringo protein was found to be essential for Xenopus oocyte maturation and a human homolog (Spy1, herein called Speedy/ Ringo A1) regulates S-phase entry and cell survival after DNA damage in cultured somatic cells. We have identified a Speedy/Ringo-like gene in the most primitive branching clade of chordates (Ciona intestinalis), as well as four mammalian homologs. Of the mammalian proteins, two, Speedy/Ringo A and C, bind to Cdc2 and Cdk2, whereas Speedy/Ringo B binds preferentially to Cdc2. Despite their distinct CDK-binding preferences, both Speedy/Ringo A and B can promote the maturation of Xenopus oocytes and all three Speedy/Ringo proteins can bind to and activate CDKs in vivo. These mammalian Speedy/Ringo proteins exhibit distinct tissue expression patterns, though all three are enriched in testis, consistent with the initial observation that Xenopus Speedy/Ringo functions during meiosis. Speedy/Ringo A is widely expressed in tissues and cell lines. Speedy/Ringo B expression appears to be testis-specific. Speedy/Ringo C is expressed in diverse tissues, particularly those that undergo polyploidization. All Speedy/Ringo proteins share a highly conserved approximately 140-aa domain we term the Speedy/Ringo box that is essential for CDK binding. Point mutations in this domain abolish CDK binding. Besides the central Speedy/Ringo box, Speedy/Ringo A contains a C-terminal portion, which promotes CDK activation, and an N-terminal portion, which is dispersible for both CDK binding and activation but that influences protein expression. The existence of this growing family of CDK activators suggests that Speedy/Ringo proteins may play as complex a role in cell cycle control as the diverse family of cyclins.