Ligand Binding Reveals a Role for Heme in Translationally-Controlled Tumor Protein Dimerization

Abstract

The translationally-controlled tumor protein (TCTP) is a highly conserved, ubiquitously expressed, abundant protein that is broadly distributed among eukaryotes. Its biological function spans numerous cellular processes ranging from regulation of the cell cycle and microtubule stabilization to cell growth, transformation, and death processes. In this work, we propose a new function for TCTP as a “buffer protein” controlling cellular homeostasis. We demonstrate that binding of hemin to TCTP is mediated by a conserved His-containing motif (His76His77) followed by dimerization, an event that involves ligand-mediated conformational changes and that is necessary to trigger TCTP's cytokine-like activity. Mutation in both His residues to Ala prevents hemin from binding and abrogates oligomerization, suggesting that the ligand site localizes at the interface of the oligomer. Unlike heme, binding of Ca2+ ligand to TCTP does not alter its monomeric state; although, Ca2+ is able to destabilize an existing TCTP dimer created by hemin addition. In agreement with TCTP's proposed buffer function, ligand binding occurs at high concentration, allowing the “buffer” condition to be dissociated from TCTP's role as a component of signal transduction mechanisms.

The Translational Controlled Tumour Protein TCTP: Biological Functions and Regulation

“…4.1) and mapped the Ca 2+ -binding site to a conserved part of the β-stranded core domain, close to the 'hinge' region that connects it to the α-helical domain. This result was consolidated and further refined in a very recent study (Lucas et al 2014). TCTP/fortilin has been reported to act as Ca 2+ -scavenger, and this was proposed as a yet another mechanism of its antiapoptotic activity (Graidist et al 2007).…”

“…Indeed, overexpression of TCTP has been shown to protect cells against Ca 2+ -dependent apoptosis induced by thapsigargin (Graidist et al 2007;Bommer et al 2010), a reagent that inhibits the Ca 2+ -pump of the ER membrane, resulting in a significant increase in cytosolic Ca 2+ -levels. The notion of TCTP acting as a Ca 2+ -scavenger, or more generally as a 'buffer-like' molecule for Ca 2+ , is consistent with its high abundance and with the fact that its binding site is rather weak (Lucas et al 2014). The importance of such a Ca 2+ -buffer-like function for TCTP was described for trophoblast cells of the placenta, where it is involved in Ca 2+ -handling and in its provision for the foetal blood circulation (Arcuri et al 2005).…”

“…A recent detailed study on ligand binding to human TCTP revealed that binding of haemin to TCTP resulted in a conformational change of the protein and promoted its dimerisation. In contrast, Ca 2+ -binding resulted in destabilisation of TCTP dimers formed in the presence of haemin (Lucas et al 2014). The authors propose a 'buffer-like' function for TCTP, since it is able to bind both haemin and Ca 2+ at higher concentrations.…”

“…Later, it was shown that TCTP levels are regulated in response to perturbations of intracellular Ca 2+ -homeostasis (Xu et al 1999). Although TCTP does not have a canonical Ca 2+ -binding site , its Ca 2+ -binding activity has been confirmed by several laboratories using a range of methods (Feng et al 2007a;Graidist et al 2007;Kim et al 2000;Lucas et al 2014;Arcuri et al 2004Arcuri et al , 2005Sanchez et al 1997). However, the different approaches used to map the Ca 2+ -binding site at the TCTP protein yielded different results (Feng et al 2007a;Graidist et al 2007;Kim et al 2000).…”

“…This paper demonstrated that TCTP reacts with artemisinin in situ and in vitro in the presence of haemin and that it binds haemin itself. The interactions of Pf-TCTP with artemisinin (Chae et al 2006) and of human TCTP with haemin (Lucas et al 2014) were subsequently studied in more detail, as was the subcellular location and calcium-binding activity in Plasmodium (Bhisutthibhan et al 1999).…”