Xenopus embryo blastomere immunostained for CPEB and tubulin. Courtesy of Joel D. Richter.

The best way to ensure the safe delivery of a protein is to produce it as close as possible to where it's needed. Reporting in Cell, Joel D. Richter and colleagues describe how this may be done for cyclin B1. By localizing the proteins responsible for translation of cyclin B1 messenger RNA to the mitotic spindle, production of this key player in cell division can be tightly controlled. And the results indicate that this regulation may be necessary for integrity of the mitotic apparatus.

Translation of cyclin B1 mRNA is regulated by cytoplasmic polyadenylation. This process is critical for the activation of different maternally inherited mRNAs during early development in many animals. It has been extensively studied in Xenopus oocytes where, in response to progesterone stimulation, the poly(A) tails of certain mRNAs (encoding, among them, several cyclins) are elongated.

A central player in polyadenylation is the cytoplasmic polyadenylation element-binding protein ( CPEB), which recruits a factor that promotes the interaction between poly(A) polymerase and the end of the mRNA. Polyadenylation in turn triggers translation, and a key to this switch is maskin — a protein that was initially identified on the basis of its specific immunoprecipitation with CPEB. Maskin seems to regulate the accessibility of the translation initiation factor eIF4E to a second factor eIF4G, which positions the 40S ribosomal subunit at the 5′ end of the mRNA.

Given that some mRNAs are concentrated in certain regions of Xenopus oocytes, Richter and colleagues wondered whether CPEB and maskin might be involved in mRNA localization as well as translation. To test this, they immunostained Xenopus oocytes at various stages of development with antibodies against the two proteins and found that both were especially concentrated at the cortex of the animal pole during late stages of oocyte development. Surprisingly, though, in the early embryo CPEB and maskin seemed to localize to structures resembling the mitotic spindle.

Closer analysis using antibodies against α-tubulin, CPEB and maskin confirmed this suspicion. At metaphase, the authors observed a gradient of CPEB and maskin along the length of the spindles, peaking at the area around the centrosomes. Microtubule spin-down assays confirmed that these proteins associate directly with microtubules in vitro, a binding event that could be disrupted by two small internal deletions in CPEB. These deletions also disrupted the localization of CPEB in vivo.

So CPEB and maskin localize to spindles and centromeres, but what about the mRNAs that they regulate? Using in situ hybridization, the authors next showed that cyclin B1 mRNA is also localized to the animal pole of Xenopus oocytes and, more specifically, to spindles. This localization depends on CPEB, as cyclin synthesis was blocked in one-cell embryos injected with an antibody against CPEB. This treatment also caused embryos to divide three to five times more slowly than controls, and many of them showed spindle defects such as multiple centrosomes, centrosomes detached from the spindles, or bifurcated spindles.

The conclusion, then, is that CPEB and maskin regulate the translation of cyclin B1, a process that is important not only for integrity of the mitotic apparatus, but for cell division as a whole. Local delivery, it seems, really is the safest option.