Differential Function of PTPα and PTPα Y789F in T Cells and Regulation of PTPα Phosphorylation at Tyr-789 by CD45

Abstract

CD45 is a major membrane protein tyrosine phosphatase (PTP) expressed in T cells where it regulates the activity of Lck, a Src family kinase important for T cell receptor-mediated activation. PTP␣ is a more widely expressed transmembrane PTP that has been shown to regulate the Src family kinases, Src and Fyn, and is also present in T cells. Here, PTP␣ was phosphorylated at Tyr-789 in CD45؊ T cells but not in CD45 ؉ T cells suggesting that CD45 could regulate the phosphorylation of PTP␣ at this site. Furthermore, CD45 could directly dephosphorylate PTP␣ in vitro. Expression of PTP␣ and PTP␣-Y789F in T cells revealed that the mutant had a reduced ability to decrease Fyn and Cbp phosphorylation, to regulate the kinase activity of Fyn, and to restore T cell receptor-induced signaling events when compared with PTP␣. Conversely, this mutant had an increased ability to prevent Pyk2 phosphorylation and CD44-mediated cell spreading when compared with PTP␣. These data demonstrate distinct activities of PTP␣ and PTP␣-Y789F in T cells and identify CD45 as a regulator of PTP␣ phosphorylation at tyrosine 789 in T cells. Protein tyrosine phosphatases (PTPs) 5, together with protein tyrosine kinases, are responsible for controlling the level of tyrosine phosphorylation within cells. In general, basal tyrosine phosphorylation levels in unstimulated cells are relatively low, suggesting that protein tyrosine phosphatase activity prevails over tyrosine kinase activities under resting conditions. Stimulation of cells via a variety of receptors linked to tyrosine kinases can induce tyrosine phosphorylation cascades that lead to cell activation or modulation of cellular function. In T lymphocytes, CD45 is a major protein tyrosine phosphatase expressed at the T cell membrane where it is thought to contribute over 90% of the membrane-associated PTP activity (1). Recognition of antigen by the T cell receptor (TCR) results in induction of a tyrosine phosphorylation cascade leading to T cell activation and is driven primarily by the activity of the Src family kinase (SFK), Lck (2). Counter-intuitively, CD45 is required for the efficient progression of this TCR-induced tyrosine phosphorylation signal, as it is required to dephosphorylate the negative regulatory site (Tyr-505) of Lck (reviewed in Refs. 3-5). This relieves inhibitory constraints on Lck and primes it for subsequent autophosphorylation at tyrosine 394 and activation. This activating function of CD45 and its requirement for efficient TCR signaling are well established. However, evidence from CD45 Ϫ/Ϫ thymocytes and certain CD45 Ϫ T cell lines indicates that the absence of CD45 can result in enhanced Lck phosphorylation at both the negative regulatory and autophosphorylation sites, suggesting a role for CD45 in both the activation and inactivation of Lck (4, 6, and reviewed in Ref. 7). Thus the function of CD45 may be to keep Lck under control, allowing activation, but preventing sustained activation or inactivation. It has been difficult to examine the role of CD45 in inactivati...

Protein tyrosine phosphatases: regulatory mechanisms

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Phosphotyrosine signaling analysis of site-specific mutations on EGFRvIII identifies determinants governing glioblastoma cell growth

“…It is worth noting that protein tyrosine phosphatase receptor type alpha (PTPRA) Y798 was also included in the reduced model. Interestingly, this phosphorylation site has been shown to bind Grb2 and to regulate the ability of PTPRA to dephosphorylate and activate Src and Fyn 30-32 . Together, the Fyn and PTPRa phosphorylation sites implicate a potential role for Src-family kinases in regulating glioma cell growth in U87-EGFRvIII.…”

Protein-tyrosine Phosphatase α Regulates Stem Cell Factor-dependent c-Kit Activation and Migration of Mast Cells

“…A role for PTP␣ has been recently demonstrated in T cell receptor-mediated activation and CD44-mediated spreading of T cells (17,21). To further our understanding of PTP␣ functions in immune cells, and also to query whether PTP␣ regulates SFK-dependent signaling that is initiated by a receptor with intrinsic enzymatic activity, we have investigated the role of PTP␣ in mast cells, and specifically in regulating signaling by the receptor-tyrosine kinase c-Kit.…”