Parasite calreticulins: Structure, C1q binding and dual carbohydrate/peptide interaction properties

Abstract

Calreticulin (CRT) is a multifaceted protein of 45k Da, early discovered as a receptor of the collagen-like regions of the complement C1q protein and better known as an ER chaperone protein, essential in calcium metabolism. Various implications in cancer, early development and immunology were discovered more recently for CRT, as well as its role as a dominant “eat-me” prophagocytic signal. Intriguingly, cell-surface exposure/secretion of CRT is among the infective strategies used by several human parasites. We focused on the CRTs from Trypanosoma cruzi (TcCRT) and Entamoeba histolytica (EhCRT). Analysis of the interaction between C1q and both parasite CRTs provided several lines of evidence for the main contribution of the C1q and CRT globular regions, at least in vitro. Because of the inherent flexibility of CRTs, their analysis by X-ray crystallography requires the design of recombinant constructs suitable for crystallization. Based on the strategy used for human and mouse CRTs, we have solved the X-ray structures of TcCRT and EhCRT. Although the hypothesis that CRT can exhibit both open and closed conformations has been proposed in relation to its chaperone function, only the open conformation has yet been observed in crystal structures. We provide the first evidence of a closed conformer in the case of EhCRT, which involves a complex conformational transition. SAXS experiments also provided additional information about the flexibility of TcCRT in solution, thus complementing crystallographic data on the open conformation. Finally, regarding the conserved lectin domain structure and chaperone function, we now provide direct evidence of its unusual hybrid recognition properties, with several examples of dual carbohydrate and peptide binding. Comparison of the structures of phylogenetically distant parasite CRTs with their mammalian counterparts highlights key features involved in their common structure and chaperone function. In addition, these structures reveal species specific structural features that might be harnessed to fight against the parasites without affecting the functions of the host CRT.