A new subfamily of high molecular mass CDC2-related kinases with PITAI/VRE motifs

Kinases of the CDC2 family play a key role in cell cycle regulation and gene expression. In the present work, we identified sea urchin and human cDNAs encoding homologues of a high molecular mass CDC2-like kinase (designated CDC2L5) sharing respectively a PITAVRE and PITAIRE motif. The human cDNA encodes the full-length amino acid sequence of the cholinesterase-related cell division controller (CHED) kinase, a previously published partial coding sequence. CDC2L5 overexpressed in mammalian cells is an approximately 170-kDa nuclear protein. The mRNA is present during the sea urchin early embryogenesis and is ubiquitously expressed in human tissues.

No effects of DC and 60-Hz AC magnetic fields on the first mitosis of two species of sea urchin embryos

The early divisions of sea urchin eggs was used as a model to study the effects of static and of 60 Hz sinusoidal magnetic fields. Two species were used (Sphaerechinus granularis and Paracentrotus lividus). Eggs were fertilized and exposed in two separate coils to the fields (up to 8 mT). Great care was taken to control the temperature of each sample. No difference was found in the time of the first division that could not be attributed to a temperature difference between samples. Comparison is made with other published data on various species.

The C2 cytosolic loop of adenylyl cyclase interacts with the activated form of G alpha s

Using the yeast two-hybrid system, we studied the physical interaction between the complete C1 and C2 cytosolic domains of Xenopus laevis type 9 (xl9C1, xl9C2) and the C2 domain of rat type 6 (r6C2) adenylyl cyclase (AC). Heterodimerization between xl9C1 and xl9C2 and homodimerization between C2 (but not C1) domains was observed. Interaction between C2 and human G alpha s (hG alpha s) was also detected and was dependent on G alpha s activation. In contrast X. laevis G alpha s (xlG alpha s), which is 92% identical to hG alpha s, was unable to interact with any of the three AC cytosolic domains tested, corroborating previous findings that showed no effector activation. Through the construction of chimeras, we demonstrated that the amino-terminal half of xlG alpha s was responsible for the lack of interaction with AC. Chimeras between mouse G alpha i2 and G alpha s (N-mG alpha i2/C-G alpha s), that have previously shown to activate AC to a higher extent than wild-type G alpha s, also interacted with the C2 cytosolic domain and with a higher affinity. Interestingly, N-mG alpha i2/C-xlG alpha s chimera was not only able to interact with C2 but also with the C1 cytosolic domain.

Cdk2 activity is dispensable for the onset of DNA replication during the first mitotic cycles of the sea urchin early embryo

Earlier work reported the important role of Cdk2 as a regulator of DNA replication in somatic cells and in Xenopus extracts. In the present report we analyze in vivo the involvement of Cdk2 in DNA replication during early embryogenesis using the first mitotic cycles of sea urchin embryos. Unfertilized Sphaerechinus granularis eggs are arrested after the second meiotic cytokinesis. Fertilization resumes the block and induces DNA replication after a short lag period, making sea urchin early embryo a good model for studying in vivo the onset of DNA replication. We show that Cdk2 as well as its potential partner cyclin A are present in the nucleus in G1 and S phase and therefore available for DNA replication. In accordance with data obtained in Xenopus egg extracts we observed that Cdk2 kinase activity is low and stable during the entire cycle. However, in contrast with this in vitro system in which Cdk2 activity is required for the onset of DNA replication, the specific inhibition of Cdk2 kinase by microinjection of the catalytically inactive Cdk2-K33R or the inhibitor p21(Cip1) does not prevent DNA replication. Because olomoucine, DMAP, and emetine treatments did not preclude DNA synthesis, neither cyclin A/Cdk1 nor cyclin B/Cdk1 kinase activities are necessary to replace the absence of Cdk2 kinase in promoting DNA replication. These data suggest that during early embryogenesis Cdks activities, in particular Cdk2, are dispensable in vivo for the initiation step of DNA replication. However, the specific localization of Cdk2 in the nucleus from the beginning of M phase to the end of S phase suggests its involvement in other mechanisms regulating DNA replication such as inhibition of DNA re-replication and/or that its regulating role is achieved through a pathway independent of the kinase activity. We further demonstrate that even after inhibition of Cdk activities, the permeabilization of the nuclear membrane is required to allow a second round of DNA replication. However, in contrast to Xenopus egg extracts, re-replication can take place in the absence of DMAP-sensitive kinase.

Developmental regulation of elongation factor-1 delta in sea urchin suggests appearance of a mechanism for alternative poly(A) site selection in gastrulae

Elongation factor-1 delta gene expression was analyzed during sea urchin development. EF-1 delta mRNA is present as a single 2.7-kb transcript in unfertilized eggs and in rapidly dividing cleavage stage embryos. It decreases rapidly 6 h after fertilization and then reappears at the gastrula stage as two transcripts of 2.7 and 2.0 kb. cDNA clones encoding the 2.7- and 2.0-kb transcripts were isolated from a sea urchin embryos library. The two cDNAs originate from alternative poly(A) site selection from a unique precursor. Both cDNAs are terminated by a poly(A) tail and were shown to encode for the same protein identified as EF-1 delta. Thus, EF-1 delta gene expression undergoes developmental regulation in early embryos leading to the presence of two poly(A) forms of the transcript. Since the 2.0-kb polyadenylated form of the EF-1 delta transcript appears at gastrula stage, our results suggest that a mechanism for alternative poly(A) site selection of the EF-1 delta transcript appears during embryonic development.

A presumptive developmental role for a sea urchin cyclin B splice variant

We show that a splice variant-derived cyclin B is produced in sea urchin oocytes and embryos. This splice variant protein lacks highly conserved sequences in the COOH terminus of the protein. It is found strikingly abundant in growing oocytes and cells committed to differentiation during embryogenesis. Cyclin B splice variant (CBsv) protein associates weakly in the cell with Xenopus cdc2 and with budding yeast CDC28p. In contrast to classical cyclin B, CBsv very poorly complements a triple CLN deletion in budding yeast, and its microinjection prevents an initial step in MPF activation, leading to an important delay in oocyte meiosis reinitiation. CBsv microinjection in fertilized eggs induces cell cycle delay and abnormal development. We assume that CBsv is produced in growing oocytes to keep them in prophase, and during embryogenesis to slow down cell cycle in cells that will be committed to differentiation.