Protein-tyrosine phosphatase TULA-2 has been shown to regulate receptor signaling in several cell types, including platelets. Platelets are critical for maintaining vascular integrity; this function is mediated by platelet aggregation in response to recognition of the exposed basement membrane collagen by the GPVI receptor, which is non-covalently associated with the signal-transducing FcR␥ polypeptide chain. Our previous studies suggested that TULA-2 plays an important role in negatively regulating signaling through GPVI-FcR␥ and indicated that the tyrosine-protein kinase Syk is a key target of the regulatory action of TULA-2 in platelets. However, the molecular basis of the down-regulatory effect of TULA-2 on Syk activation via FcR␥ remained unclear. In this study, we demonstrate that suppression of Syk activation by TULA-2 is mediated, to a substantial degree, by dephosphorylation of Tyr(P) 346 , a regulatory site of Syk, which becomes phosphorylated soon after receptor ligation and plays a critical role in initiating the process that yields fully activated Syk. TULA-2 is capable of dephosphorylating Tyr(P) 346 with high efficiency, thus controlling the overall activation of Syk, but is less efficient in dephosphorylating other regulatory sites of this kinase. Therefore, dephosphorylation of Tyr(P) 346 may be considered an important "checkpoint" in the regulation of Syk activation process. Putative biological functions of TULA-2-mediated dephosphorylation of Tyr(P) 346 may include deactivation of receptor-activated Syk or suppression of Syk activation by suboptimal stimulation.Proteins of the two-member UBASH3/STS/TULA family featuring the histidine phosphatase domain in their structure act as regulators of signaling and other cellular processes in several experimental systems (1-11) (reviewed in Refs. 12-14).TULA-2, also termed p70, STS-1, or UBASH3B, has been shown to be an active protein-tyrosine phosphatase (PTP) 2 (15-17) capable of suppressing receptor signaling in T lymphocytes (1, 7, 9, 15), mast cells (18), osteoclasts (8), and platelets (6,19,20).Platelets play a key role in maintaining vascular integrity. Platelets circulate in the bloodstream in a quiescent discoid shape until they interact with collagen, an essential part of the extracellular matrix, which becomes exposed upon blood vessel injury. This interaction triggers signaling, which activates platelets and causes their aggregation, thus developing a plug to arrest bleeding (21, 22). However, this trigger-like response can also have deleterious consequences if left unchecked. Therefore, negative regulation of platelet signaling is critical for maintaining a balance between hemostasis and thrombosis (23).Our previous studies suggested that TULA-2 plays an important role in negatively regulating platelet signaling through GPVI, a receptor for collagen non-covalently associated with Fc⑀R1␥ (denoted FcR␥ throughout), and Fc␥RIIA, an IgG Fc receptor, both of which transmit signals through the immunoreceptor tyrosine-based activation motifs (ITA...

No abstract