The serine phosphatases PP1 and PP2A associate with and activate the actin-binding protein cofilin in human T lymphocytes

Abstract

Cofilin, an actin‐depolymerizing protein, is essential for the functional dynamics of the actin cytoskeleton and for cell viability. In unstimulated human peripheral blood T lymphocytes cofilin is phosphorylated and localized in the cytoplasm. Following co‐stimulation through accessory receptors (e.g. CD2 or CD28) – however, not following TCR/CD3 stimulation alone – cofilin undergoes dephosphorylation. The subcellular localization as well as the actin‐binding activity of cofilin are regulated by the phosphorylation state of serine‐3. Thus, only the dephosphorylated form of cofilin associates with the actin cytoskeleton and possesses the capability to translocate into the nucleus. Recently, LIM‐kinase 1 was shown to inactivate cofilin through phosphorylation. Here, we have identified the functional counterparts of LIM‐kinase 1: the serine/threonine phosphatases of type 1 and type 2A not only associate with cofilin but also dephosphorylate this 19‐kDa protein and thereby mediate cofilin activation. In malignant T lymphoma cells, activation of these phosphatases occurs spontaneously, independent of external stimuli. In untransformed human peripheral blood T lymphocytes, these phosphatases function through a cyclosporin A/FK506‐resistant co‐stimulatory signaling pathway which is common for the accessory receptors CD2 and CD28. This co‐stimulatory signaling pathway is also not affected by a series of other clinically established immunosuppressive drugs (i.e. rapamycin, dexamethasone, leflunomide or mycophenolic acid).

p21-Activated Kinase 2 in Neutrophils Can Be Regulated by Phosphorylation at Multiple Sites and by a Variety of Protein Phosphatases

“…The assay conditions for PP 2B (calcineurin) were identical to those described above but also included 0.20 mM CaCl 2 and 70 g/ml calmodulin (34). The assay conditions for PP 2C were identical to those for PP 1 and PP 2A but in this case MnCl 2 was replaced by 5.0 mM MgCl 2 (e.g., Ref.…”

“…Thus, this fusion protein may serve as a substrate to gain information on potential phosphatases that recognize phosphorylated residues in the AL and ID of Pak. GST-Pak 3 was treated with purified PP 1␣, PP 2A, PP 2B, and PP 2C for 15 min at 37°C under conditions known to be effective for each of these phosphatases (34) and phosphorylation at Thr 421 and Ser 139 was monitored with the pAbs described above (Fig. 7).…”

The cofilin pathway in breast cancer invasion and metastasis

“…Cofilin is then regulated by four independent processes. First, the phosphorylation of cofilin on serine 3 by LIM kinase 1 (LIMK1) and its related kinases (LIMK2, the skeletal muscle-specific kinases Nik related kinase (NRK, also known as NESK) and testicular protein kinase 1 (TESK1) and TESK2) regulates cofilin by inhibiting its actin-binding activity [19][20][21][22] ; second, the dephosphorylation of serine 3 of cofilin by phosphatase types 1, 2A and 2B, slingshot (SSH) and chronophin phosphatases results in the activation of actin binding by cofilin [23][24][25][26] . Third, actin binding by cofilin is inhibited by binding to phosphatidylinositol-4,5-bisphosphate (PIP2) 27,28 , and cofilin activity is dependent on phospho lipase Cγ (PLCγ)-mediated hydrolysis of PIP2 (REFS 29,30); and fourth, changes in pH over the physiological range (6.8-7.4) as mediated by the Na-H exchanger protein can activate the severing activity of cofilin when it is in the dephosphorylated state [31][32][33][34] …”

Signaling mechanisms that regulate actin‐based motility processes in the nervous system

“…There is evidence to suggest this occurs in response to growth factors (see below). Several potential activating phosphatases have been identified, including PP1, PP2A, and PP2B (Meberg et al 1998;Ambach et al 2000). Slingshot (SSH) is a recently discovered family of phosphatases that have apparent specificity for ADF/cofilin (Niwa et al 2002).…”