Dual-Specific Protein and Lipid Phosphatase PTEN and Its Biological Functions

Abstract

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) encodes a 403-amino acid protein with an amino-terminal domain that shares sequence homology with the actin-binding protein tensin and the putative tyrosine-protein phosphatase auxilin. Crystal structure analysis of PTEN has revealed a C2 domain that binds to phospholipids in membranes and a phosphatase domain that displays dual-specific activity toward both tyrosine (Y), serine (S)/threonine (T), as well as lipid substrates in vitro. Characterized primarily as a lipid phosphatase, PTEN plays important roles in multiple cellular processes including cell growth/survival as well as metabolism.

Figure 1.

Structure of catalytic pocket of PTEN defines it as a dual protein phosphatase and a phosphoinositide phosphatase. The CX5R motif on protein tyrosine phosphatases forms a P-loop that sits at the catalytic active pockets. A cysteine at the base of this pocket is required for the phosphatase to react with the substrate. (Top) Structure of the catalytic pocket of PTP1B, a protein tyrosine phosphatase also contains Q-loop (left), WPD-loop (top), E-loop (right), and pTyr-loop (bottom) (labeled in green). A conserved glutamine on the Q-loop position a water molecule for hydrolysis. E-loop functions to regulate the dynamics of the WPD-loop. pTyr-loop controls specificity of classical PTP substrates. (Bottom) In addition to P-loop and WPD-loop that are also found on PTP1B, PTEN also contains a TI-loop and a pβ2-α1-loop. TI-loop determines the size of the catalytic pocket of PTEN and allowed accommodation of large lipids such as phosphatidylinositol 3,4,5-triphosphate (PIP₃). Function of pβ2-α1-loop is not clear but it may have a similar location as pTyr-loop in classical PTPs. (Image generated using data from the Protein Data Bank and analyzed using the UCSF Chimera software.)

Figure 2.

The interaction of PTEN with its protein substrates at the catalytic pocket. The catalytic cysteine (C124) of the P-loop initiates a nucleophilic attack on the phosphorous atom of the phosphopeptide. This nucleophilic attack by C124 results in the breakage of the oxygen–phosphorus bond between the peptide and its phosphate group, leading to the formation of a phosphocysteine through a phosphorous–sulfur bond. This process is facilitated by aspartic acid (D92) of the WPD-loop, which donates a proton to the hydroxy leaving group on the substrate protein. The aspartic acid (D92) that donated a proton is left with a negatively charged oxygen atom, which now binds to the hydrogen on the water molecule. This allows the nucleophilic hydroxy group of the water molecule to hydrolyze the phosphorous–sulfur bond and release the free phosphate. A guanidinium group of a conserved arginine (R130) on the P-loop is needed to coordinate the position of the phosphate group during this process. (Data downloaded from the Protein Data Bank and analyzed using CCP4MG software.)

Figure 3.

Domain structure of the PTEN protein. PTEN is composed of five functional domains: (1) a PIP₂ (PI(4,5)P₂)-binding domain (PBD); (2) a phosphatase domain including the catalytic core (H123–R130) in which cancer-associated mutations are enriched; (3) a C2 domain, which is critical in PTEN subcellular location regulation; its interaction with the membrane-binding domain and the ubiquitination of it have both been found to regulate PTEN translocation; (4) two PEST (proline, glutamic acid, serine, and threonine) domains that are associated with degradation; and (5) a PDZ motif for protein–protein interactions. Critical point mutations that disrupt lipid or protein phosphatase activity or both are indicated with arrows.