Phosphatase in Cajal Bodies

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Science's STKE  22 Apr 2003:
Vol. 2003, Issue 179, pp. tw162-TW162
DOI: 10.1126/stke.2003.179.tw162

The Survival of Motor Neurons (SMN) protein appears to have multiple tasks in the cytoplasm and nucleus. It accompanies assembled small nuclear RNPs (snRNPs) to sites in the nucleus called Cajal bodies, where SMN is thought to control active spliceosome and transcriptosome assembly; hence, it coordinates the synthesis and processing of mRNA and the metabolism of snRNPs. SMN is part of a larger protein complex that includes Gemin2, 3, and 4, and Sm proteins, but little is known about how the SMN complex is regulated. Carnegie et al. report that a member of the PPP family of serine-threonine protein phosphatases called PPP4 is associated with Gemin3 and 4. Gemin3 is a putative adenosine triphosphate (ATP)-dependent RNA helicase implicated in RNA metabolism, and Gemin4 is a Gemin3-binding protein. The Gemin proteins coimmunoprecipitated with the regulatory and catalytic subunits of PPP4 in transfected cells. PPP4 and its regulatory subunit were found predominantly in the nucleus. Expression of the regulatory subunit also enhanced maturation of snRNPs, as revealed by the accelerated movement of fluorescently labeled Sm proteins into Cajal bodies. Gemin3 is a phosphoprotein and PPP4 could regulate its helicase activity, and thus, snRNP assembly.

G. K. Carnegie, J. E. Sleeman, N. Morrice, C. J. Hastie, M. W. Peggie, A. Philp, A. I. Lamond, P. T. W. Cohen, Protein phosphatase 4 interacts with the Survival of Motor Neurons complex and enhances the temporal localisation of snRNPs. J. Cell Sci. 116, 1905-1913 (2003). [Abstract] [Full Text]

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