CellTweets #9   A Look Inside the Nuclear Actin Black Box

Actin's role in the nucleus comes packed in its very own black box. The cell nucleus is a very small and complex place but given how much is known about actin in the cytoplasm, it can be startling to hear how little agreement there is about actin in the nucleus. Indeed, only in recent years has actin's presence in the nucleoplasm been widely conceded along with some—still to be delineated—role as an RNA processing factor. Now comes a new study in Molecular Biology of the Cell (MBoC) that sheds direct light on what actin is up to in the nucleus of somatic cells. But the work also underscores the difficulty of shedding light in very small black boxes.

One of the problem facing researchers has been crafting the proper reporter probes for tracking nuclear actin, says Brittany Belin, the first author on the paper with colleagues Beth Cimini, Elizabeth Blackburn, and R. Dyche Mullins, all of the University of California, San Francisco (USCF). Actin-binding domains (ABDs) have to be selected to differentiate between monomeric and filamentous actin. Nor can the ABD be too powerful because of another quirk of nuclear actin—it is found at very high levels in the oocyte nucleus but at very low levels in the somatic nucleus. Working in human somatic osteosarcoma cells, the design of a monomeric actin-binding probe or G (for globular) ABD was fairly straightforward, Belin explains. Designing a filamentous actin-binding domain probe or FABD was not. At such low concentrations, an overly powerful FABD might create its own artifacts by pulling filamentous actin into artificial clumps.

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