A truncated isoform of the PP2A B56 subunit promotes cell motility through paxillin phosphorylation

Abstract

Both F10 and BL6 sublines of B16 mouse melanoma cells are metastatic after intravenous injection, but only BL6 cells are metastatic after subcutaneous injection. Retrotransposon insertion was found to produce an N-terminally truncated form (Δγ1) of the B56γ1 regulatory subunit isoform of protein phosphatase (PP) 2A in BL6 cells, but not in F10 cells. We found an interaction of paxillin with PP2A C and B56γ subunits by co-immunoprecipitation. B56γ1 co-localized with paxillin at focal adhesions, suggesting a role for this isoform in targeting PP2A to paxillin. In this regard, Δγ1 behaved similarly to B56γ1. However, the Δγ1-containing PP2A heterotrimer was insufficient for the dephosphorylation of paxillin. Transfection with Δγ1 enhanced paxillin phosphorylation on serine residues and recruitment into focal adhesions, and cell spreading with an actin network. In addition, Δγ1 rendered F10 cells as highly metastatic as BL6 cells. These results suggest that mutations in PP2A regulatory subunits may cause malignant progression.

Protein kinase A activates protein phosphatase 2A by phosphorylation of the B56δ subunit

“…However, it seems likely that the N-and C-terminal regions of B56 isoforms will have additional influence on PP2A activity. For example, Nterminal deletion of B56␥1 can affect substrate specificity (30). Phosphorylation of Ser-566 of B56␦ may be able to increase PP2A affinity for DARPP-32 or other substrates through additional protein-protein interactions, or by affecting the structure of the B56 core which in turn influences the interaction of the catalytic domain with substrates.…”

Paxillin: Adapting to Change

“…Paxillin can also interact with talin (223), poly(A)-binding protein 1 (307), ASAP1/2 (128,195), the nerve growth factor (NGF) receptor TrkA (178), the antiapoptosis protein bcl-2 (254,255), the focal adhesion protein TES (69), RACK (42), the serine/threonine phosphatase PP2A (110), the tyrosine phosphatase PTP- (206), Crk and CrkL (19,225,232), the tyrosine kinases Abl (143) and p210BCR/ABL (225), the protooncogene Cbl (224), and LIM kinase (62). Paxillin and Hic-5 also bind to the syndecan binding protein syndesmos (52), Hsp27 (118), Hsp72 (172), and the ring-finger ubiquitination component Rnf5 (54).…”

“…Regardless, mutagenesis of the LIM domains that mediate PTP-PEST binding decreases cell adhesion and motility on fibronectin, perhaps indicating a role for paxillin binding to PTP-PEST in focal adhesion dynamics (27, 41). Finally, paxillin also interacts with the serine/threonine phosphatase PP2A, and although it is not known if paxillin is a physiological protein phosphatase 2A substrate, perturbation of this association and consequent increase in paxillin LIM3 serine phosphorylation enhances cell metastasis (110,323).…”

Increased activity of phosphatase PP2A in the presence of the PlA2 polymorphism of αIIbβ3

“…Central to our findings, while αIIbβ3 binding to its ligands increased the activation of phosphatase PP2A, the PlA2 polymorphism of β3 did so to a greater extent than PlA1 did. PP2A, in turn, plays a critical role in αIIbβ3-mediated cell adhesion Integrin mediated cell adhesion is known to induce changes in PP2A activity [18] and localization within integrin-organized focal adhesion complex through its interaction with paxillin [19]. Instructively, the activity of αIIbβ3 has been shown to regulate phosphatase PP1 [20].…”