TULA-Family Regulators of Platelet Activation

Abstract

The two members of the UBASH3/TULA/STS-protein family have been shown to critically regulate cellular processes in multiple biological systems. The regulatory function of TULA-2 (also known as UBASH3B or STS-1) in platelets is one of the best examples of the involvement of UBASH3/TULA/STS proteins in cellular regulation. TULA-2 negatively regulates platelet signaling mediated by ITAM- and hemITAM-containing membrane receptors that are dependent on the protein tyrosine kinase Syk, which currently represents the best-known dephosphorylation target of TULA-2. The biological responses of platelets to collagen and other physiological agonists are significantly downregulated as a result. The protein structure, enzymatic activity and regulatory functions of UBASH3/TULA/STS proteins in the context of platelet responses and their regulation are discussed in this review.

Keywords: ITAM; Syk; TULA-2; hemITAM; phosphorylation; platelet; signaling.

Figure 1

TULA-family protein structure, domains and major interactions. Major functional domains of UBASH3/STS/TULA proteins are shown, including ubiquitin-associated domain (UBA), Src-homology domain 3 (SH3) and histidine phosphatase (HP) domain. The 2H phosphoesterase domain has been identified in TULA-2. The degree of homology within major domains is shown as the percentage of similar (‘positive’) amino acid residues. Major interactions, including enzymatic activities, are outlined for various domains; most of them are characteristic of both family members. The key catalytic histidine of the histidine phosphatase domain is indicated (H380 in human TULA-2). The C-terminal sequence mediates dimerization of TULA proteins. See details in the text.

Figure 2

TULA-2 downregulates platelet signaling mediated by Syk. Major upstream events of platelet signaling through immunoreceptor tyrosine-based activation motif (ITAM)- or hemITAM (hemi-ITAM)-bearing receptors and G-protein-coupled receptors (GPCR) are schematically represented. Early signaling events through ITAM- or hemITAM-bearing receptors involve Src-family protein tyrosine kinases (SFKs) and Syk, a protein tyrosine kinase interacting with phosphotyrosines of ITAMs and hemITAMs through its tandem SH2 domains (see details in the text). This scheme illustrates how Syk, following activation through a receptor, phosphorylates its protein substrates, including LAT and SLP-76 adaptors, which interact with other signaling proteins activating PLC-γ, thus increasing the intracellular Ca²⁺ concentration. Activation through the protease-activated receptor (PAR), which is a GPCR, increases Ca²⁺ in a Syk-independent fashion. TULA-2 downregulates Syk-mediated receptor signaling by dephosphorylating Syk phosphotyrosines, which positively regulate activity of this kinase. Various events dependent on receptor-induced Syk activation are downregulated by TULA-2, not only those depicted in this figure.

Figure 3

Effect of TULA-2 on Syk regulatory phosphotyrosines. Major domains, interdomain regions and regulatory phosphotyrosines (pY) of Syk are depicted (residue numbering is for mouse Syk). pY342 and pY346 exert positive regulatory effects on Syk, while pY317 is a negative regulatory site. The pY519/pY520 site is located in the activation loop of Syk and represents a marker of Syk activation. The differential ability of TULA-2 to dephosphorylate the sites depicted here is indicated and varies from very strong (++) to strong (+) to moderate (±) to the lack thereof (−). See the text for detail.