Tankyrases (TNKS and TNKS2) are enzymes that catalyze poly-ADP-ribosylation (PARsylation) of their target proteins. Tankyrase-mediated PARsylation plays critical regulatory roles in cell signaling, particularly in the Wnt/β-catenin pathway. The sterile alpha motif (SAM) domain in tankyrases mediates their oligomerization, which is essential for tankyrase function. The oligomerization regulates the catalytic activity of tankyrases, but the underlying mechanism is unclear. Our analyses of crystal structures of the tankyrase catalytic domain suggest that formation of a head-to-head dimer regulates the catalytic activity. Our activity assays show that residues in the catalytic domain dimer interface are important for the PARsylation activity of tankyrases both in solution and cells. The dimer is weak and may only form in the context of the SAM domain-mediated oligomers of tankyrases, consistent with the dependence of the tankyrase activity on the SAM domain.

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Glycan binding often mediates extracellular macromolecular recognition events. Accurate characterization of these binding interactions can be difficult because of dissociation and scrambling that occur during purification and analysis steps. Use of photocrosslinking methods has been pursued to covalently capture glycan-dependent interactions in situ; however, use of metabolic glycan engineering methods to incorporate photocrosslinking sugar analogs is limited to certain cell types. Here, we report an exoenzymatic labeling method to add a diazirine-modified sialic acid (SiaDAz) to cell surface glycoconjugates. The method involves the chemoenzymatic synthesis of diazirine-modified CMP-sialic acid (CMP-SiaDAz), followed by sialyltransferase-catalyzed addition of SiaDAz to desialylated cell surfaces. Cell surface SiaDAzylation is compatible with multiple cell types and is facilitated by endogenous extracellular sialyltransferase activity present in Daudi B cells. This method for extracellular addition of α2-6-linked SiaDAz enables UV-induced crosslinking of CD22, demonstrating the utility for covalent capture of glycan-mediated binding interactions.

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