Research ArticleCell Biology

MARK3-mediated phosphorylation of ARHGEF2 couples microtubules to the actin cytoskeleton to establish cell polarity

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Sci. Signal.  31 Oct 2017:
Vol. 10, Issue 503, eaan3286
DOI: 10.1126/scisignal.aan3286

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MARKing the switch from microtubules to actin

To enable them to carry out specialized functions, many cell types become polarized through the activity of various proteins, including the kinases of the MARK family. Sandí et al. found that MARK3 phosphorylated Ser151 in the cytoskeleton-associated protein ARHGEF2. This phosphorylation event caused ARHGEF2 to dissociate from microtubules and activate RHOA, resulting in the formation of focal adhesions and stress fibers and enabling the formation of three-dimensional structures by cultured cells. These effects were reversed by the PP2A-mediated dephosphorylation of Ser151 in ARHGEF2. Thus, the phosphorylation state of Ser151 of ARHGEF2 determines whether it is sequestered by the tubulin cytoskeleton or released to remodel the actin cytoskeleton and regulate cell polarity.

Abstract

The PAR-1–MARK pathway controls cell polarity through the phosphorylation of microtubule-associated proteins. Rho-Rac guanine nucleotide exchange factor 2 (ARHGEF2), which activates Ras homolog family member A (RHOA), is anchored to the microtubule network and sequestered in an inhibited state through binding to dynein light chain Tctex-1 type 1 (DYNLT1). We showed in mammalian cells that liver kinase B1 (LKB1) activated the microtubule affinity-regulating kinase 3 (MARK3), which in turn phosphorylated ARHGEF2 at Ser151. This modification disrupted the interaction between ARHGEF2 and DYNLT1 by generating a 14-3-3 binding site in ARHGEF2, thus causing ARHGEF2 to dissociate from microtubules. Phosphorylation of ARHGEF2 by MARK3 stimulated RHOA activation and the formation of stress fibers and focal adhesions, and was required for organized cellular architecture in three-dimensional culture. Protein phosphatase 2A (PP2A) dephosphorylated Ser151 in ARHGEF2 to restore the inhibited state. Thus, we have identified a regulatory switch controlled by MARK3 that couples microtubules to the actin cytoskeleton to establish epithelial cell polarity through ARHGEF2.

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