Abstract
FilGAP is a Rho GTPase-activating protein (GAP) that specifically regulates Rac. FilGAP is phosphorylated by ROCK, and this phosphorylation stimulates its RacGAP activity. However, it is unclear how phosphorylation regulates cellular functions and localization of FilGAP. We found that non-phosphorylatable FilGAP (ST/A) mutant is predominantly localized to the cytoskeleton along actin filaments and partially co-localized with vinculin around cell periphery, whereas phosphomimetic FilGAP (ST/D) mutant is diffusely cytoplasmic. Moreover, phosphorylated FilGAP detected by Phos-tag is also mainly localized in the cytoplasm. Of the six potential phosphorylation sites in FilGAP tested, only mutation of serine 402 to alanine (S402A) resulted in decreased cell spreading on fibronectin. FilGAP phosphorylated at Ser-402 is localized to the cytoplasm but not at the cytoskeleton. Although Ser-402 is highly phosphorylated in serum-starved quiescent cells, dephosphorylation of Ser-402 is accompanied with the cell spreading on fibronectin. Treatment of the cells expressing wild-type FilGAP with calyculin A, a Ser/Thr phosphatase inhibitor, suppressed cell spreading on fibronectin, whereas cells transfected with FilGAP S402A mutant were not affected by calyculin A. Expression of constitutively activate Arf6 Q67L mutant stimulated membrane blebbing activity of both non-phosphorylatable (ST/A) and phosphomimetic (ST/D) FilGAP mutants. Conversely, depletion of endogenous Arf6 suppressed membrane blebbing induced by FilGAP (ST/A) and (ST/D) mutants. Our study suggests that Arf6 and phosphorylation of FilGAP may regulate FilGAP, and phosphorylation of Ser-402 may play a role in the regulation of cell spreading on fibronectin.
Highlights
FilGAP is a Rac GTPase-activating protein, but regulation via phosphorylation has not been previously characterized
Phosphorylation of FilGAP Regulates Its Subcellular Localization—We have shown previously that a FilGAP mutant with all potential phosphorylation sites mutated to alanine (ST/A) failed to function as a RacGAP in cells, whereas a FilGAP mutant with all potential phosphorylation sites mutated to phosphomimetic aspartic acid (ST/D) suppressed Rac-driven lamellae formation in vivo
We found Ͼ40% of the stimulated membrane blebbing activity of both non-phosphorylatable (ST/A) mutant in the Triton X-100 insoluble fraction, whereas Ͻ20% of the ST/D mutant was found in this fraction (Fig. 1, A and B)
Summary
FilGAP is a Rac GTPase-activating protein, but regulation via phosphorylation has not been previously characterized. FilGAP is a Rho GTPase-activating protein (GAP) that regulates Rac. FilGAP is phosphorylated by ROCK, and this phosphorylation stimulates its RacGAP activity. The cell lysates were precleared, the supernatants were collected and subjected to SDS-PAGE, and proteins were detected by immunoblot using anti-HA or anti-Ser(P)-402 antibody. The cells were washed 3 times with 2 ml of PBS, lysed by 120 l of lysis buffer (radioimmune precipitation assay buffer) containing 50 mM Tris-HCl (pH 7.4), 500 mM NaCl, 0.5% Triton X-100, 0.5% sodium deoxycholate, 0.1% SDS, 1 mM EDTA, 1 mM sodium orthovanadate, 30 mM sodium pyrophosphate, and 50 mM sodium fluoride with protease inhibitors, and centrifuged at 200,000 ϫ g for 20 min. Forty-eight hours after transfection, the levels of each protein were measured by Western blot analysis using anti-Arf or anti-FilGAP antibody
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