Verma D, Murmu A, Gourinath S, Bhattacharya A, Chary KVR. Structure of Ca2+-binding protein-6 from Entamoeba histolytica and its involvement in trophozoite proliferation regulation.
PLoS Pathog 2017;
13:e1006332. [PMID:
28505197 PMCID:
PMC5444848 DOI:
10.1371/journal.ppat.1006332]
[Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2016] [Revised: 05/25/2017] [Accepted: 04/03/2017] [Indexed: 12/01/2022] Open
Abstract
Cell cycle of Entamoeba histolytica, the etiological agent of amoebiasis, follows a novel pathway, which includes nuclear division without the nuclear membrane disassembly. We report a nuclear localized Ca2+-binding protein from E. histolytica (abbreviated hereafter as EhCaBP6), which is associated with microtubules. We determined the 3D solution NMR structure of EhCaBP6, and identified one unusual, one canonical and two non-canonical cryptic EF-hand motifs. The cryptic EF-II and EF-IV pair with the Ca2+-binding EF-I and EF-III, respectively, to form a two-domain structure similar to Calmodulin and Centrin proteins. Downregulation of EhCaBP6 affects cell proliferation by causing delays in transition from G1 to S phase, and inhibition of DNA synthesis and cytokinesis. We also demonstrate that EhCaBP6 modulates microtubule dynamics by increasing the rate of tubulin polymerization. Our results, including structural inferences, suggest that EhCaBP6 is an unusual CaBP involved in regulating cell proliferation in E. histolytica similar to nuclear Calmodulin.
E. histolytica, the etiological agent of amoebiasis, is a protozoan parasite responsible for around 100,000 deaths per year in developing nations. Though the organism has been identified more than 100 years back, there is not much understanding about the biology of this organism. Calcium signaling plays an important role in the biology of this organism. Here we show structure-functional relationship of one of the Ca2+-binding proteins (abbreviated as EhCaBP6) and suggest its involvement in cell division in this parasite. EhCaBP6, a nucleo-cytosolic Ca2+-binding protein, is a microtubule end binding protein and overexpression of its gene induces an increase in number of microtubular assemblies in E. histolytica. Cell division cycle in E. histolytica occurs along the microtubular structures without disruption of nuclear envelope. Occurrence of multinucleated cells in culture suggests duplication and reduplication of nuclear DNA without cytokinesis. Although Kinesin like protein (Klp1), Formin1 and EhCaBP6 were shown to be part of the microtubular assembly, their role in regulation of the cell cycle is not yet documented. Further, E. histolytica does not have a typical CaM like protein. However, the 3D structure of EhCaBP6 with two Ca2+-binding sites is similar to CaM, in spite of their low sequence similarity. Here, we demonstrate that EhCaBP6 regulates cell cycle specifically by facilitating DNA synthesis, transition from G1 to S phase and cytokinesis. The structural and functional similarity between EhCaBP6 and CaM suggests EhCaBP6 to be a functional homologue of nuclear CaM with important roles in regulation of cell cycle.
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