Liu, J., Prickett, T.D., Elliott, E., Meroni, G. & Brautigan, D.L. Phosphorylation and microtubule association of the Opitz syndrome protein mid-1 is regulated by protein phosphatase 2A via binding to the regulatory subunit 4. Proc. Natl Acad. Sci. USA 98, 6650-6655

Center for Cell Signaling, University of Virginia School of Medicine, P.O. Box 800577, Charlottesville, VA 22908-0577, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 07/2001; 98(12):6650-5. DOI: 10.1073/pnas.111154698
Source: PubMed


Opitz syndrome (OS) is a human genetic disease characterized by deformities such as cleft palate that are attributable to defects in embryonic development at the midline. Gene mapping has identified OS mutations within a protein called Mid1. Wild-type Mid1 predominantly colocalizes with microtubules, in contrast to mutant versions of Mid1 that appear clustered in the cytosol. Using yeast two-hybrid screening, we found that the alpha4-subunit of protein phosphatases 2A/4/6 binds Mid1. Epitope-tagged alpha4 coimmunoprecipitated endogenous or coexpressed Mid1 from COS7 cells, and this required only the conserved C-terminal region of alpha4. Localization of Mid1 and alpha4 was influenced by one another in transiently transfected cells. Mid1 could recruit alpha4 onto microtubules, and high levels of alpha4 could displace Mid1 into the cytosol. Metabolic (32)P labeling of cells showed that Mid1 is a phosphoprotein, and coexpression of full-length alpha4 decreased Mid1 phosphorylation, indicative of a functional interaction. Association of green fluorescent protein-Mid1 with microtubules in living cells was perturbed by inhibitors of MAP kinase activation. The conclusion is that Mid1 association with microtubules, which seems important for normal midline development, is regulated by dynamic phosphorylation involving MAP kinase and protein phosphatase that is targeted specifically to Mid1 by alpha4. Human birth defects may result from environmental or genetic disruption of this regulatory cycle.

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    • "Alpha4 was shown to bind and induce a conformational change that inactivated the PP2Ac subunit, and this complex apparently protected the PP2Ac subunit from degradation [4], [21]. On the other hand, alpha4 is postulated to facilitate a MID1-alpha4-PP2Ac complex that promoted the ubiquitination of PP2Ac by MID1 [17], [24], [37]. Our ubiquitination data suggest that MID1 could interact with and target PP2Ac in the absence of alpha4 (Figure 2A). "
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    • "The amount of Tap42 associated PP2A/C was predicted to be about 2% of the whole PP2A/C pool. However, there were only 28% shared sequence identity between α4 and Tap42, thus the amount of α4 associated PP2A/C was still unknown to our knowledge 12. It is notable that the complex of α4 and PP2A/C is independent of the A and B subunits, as the binding sites between α4 and PP2A/C partially overlap with that of the PP2A A and C subunits 13. "
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    • "These observations provide additional support for the critical role of primary cilia in Gli3 regulation and suggest that GLI3 is involved in the pathogenesis of OFDS. GLI3 is also linked to Opitz syndrome (OS), a genetically heterogeneous disorder characterized by defects of the ventral midline (Liu et al. 2001). "
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