Abstract
PTEN prevents tumor genesis by antagonizing the PI3 kinase/Akt pathway through D3 site phosphatase activity toward PI(3,4)P2 and PI(3,4,5)P3. The structural determinants of this important specificity remain unknown. Interestingly, PTEN shares remarkable homology to voltage-sensitive phosphatases (VSPs) that dephosphorylate D5 and D3 sites of PI(4,5)P2, PI(3,4)P2, and PI(3,4,5)P3. Since the catalytic center of PTEN and VSPs differ markedly only in TI/gating loop and active site motif, we wondered whether these differences explained the variation of their substrate specificity. Therefore, we introduced mutations into PTEN to mimic corresponding sequences of VSPs and studied phosphatase activity in living cells utilizing engineered, voltage switchable PTENCiV, a Ci-VSP/PTEN chimera that retains D3 site activity of the native enzyme. Substrate specificity of this enzyme was analyzed with whole-cell patch clamp in combination with total internal reflection fluorescence microscopy and genetically encoded phosphoinositide sensors. In PTENCiV, mutating TI167/168 in the TI loop into the corresponding ET pair of VSPs induced VSP-like D5 phosphatase activity toward PI(3,4,5)P3, but not toward PI(4,5)P2. Combining TI/ET mutations with an A126G exchange in the active site removed major sequence variations between PTEN and VSPs and resulted in D5 activity toward PI(4,5)P2 and PI(3,4,5)P3 of PTENCiV. This PTEN mutant thus fully reproduced the substrate specificity of native VSPs. Importantly, the same combination of mutations also induced D5 activity toward PI(3,4,5)P3 in native PTEN demonstrating that the same residues determine the substrate specificity of the tumor suppressor in living cells. Reciprocal mutations in VSPs did not alter their substrate specificity, but reduced phosphatase activity. In summary, A126 in the active site and TI167/168 in the TI loop are essential determinants of PTEN’s substrate specificity, whereas additional features might contribute to the enzymatic activity of VSPs.
Highlights
PTEN is a well-characterized D3-phosphoinositide (PI) phosphatase that dephosphorylates PI(3,4)P2 and PI(3,4,5)P3 to PI(4)P and PI(4,5)P2, respectively [1,2,3,4,5,6]
We found that alanine at position 126 in the P loop and the TI pair in the TI loop independently determine substrate specificity of PTENCiV: exchanging these amino acids individually to corresponding residues of voltage-sensitive phosphatases (VSPs) conferred VSP-like D5 activity toward PI(3,4,5)P3 to otherwise D3 site-specific PTENCiV, the phosphatase remained inactive toward PI(4,5)P2
Following established techniques to assess the substrate specificity of voltage-sensitive PI phosphatases in living cells [23, 25, 37], we expressed the enzymes together with GFPtagged PI-binding domains in Chinese hamster ovary (CHO) cells
Summary
PTEN (phosphatase and tensin homologue deleted on chromosome 10) is a well-characterized D3-phosphoinositide (PI) phosphatase that dephosphorylates PI(3,4)P2 and PI(3,4,5)P3 to PI(4)P and PI(4,5)P2, respectively [1,2,3,4,5,6]. PTEN shows high sequence similarity to the catalytic domain (CD) of voltage-sensitive phosphatases (VSPs; Fig. S1), such as CiVSP [18], Dr-VSP [19], Xl-VSP1 and Xl-VSP2 [20], and the human or thologue Hs-VSP1 (previously named hVSP1, TPTE2 or TPIP) [21,22,23]. Recent data expanded the initial concept of VSPs being pure D5 site-specific phosphatases, since D3 activity toward PI(3,4)P2 and PI(3,4,5)P3 was revealed for Ci-VSP [26,27,28,29,30]. These findings indicate less specific activity of VSPs compared to the highly specific D3 phosphatase PTEN.
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