Ca2+-binding protein from Entamoeba histolytica (EhCaBP6) is a novel GTPase.

GTPases are molecular switches, which regulate a variety of cellular processes such as cell polarity, gene transcription, microtubule dynamics, cell-cycle etc. In this paper, we characterize a Ca2+-binding protein from Entamoeba histolytica (EhCaBP6) as a novel GTPase. We locate the active site for GTP hydrolysis within the C-terminal domain of EhCaBP6, although it requires full length protein for its complete range of activity. Using NMR studies, we observe that GTP binding induces conformational change in EhCaBP6. The identification of this novel and unusual Ca2+-dependent GTPase is important to elucidate the unconventional cell cycle of E. histolytica.

Isoform-dependent feedback regulation of serine O-acetyltransferase isoenzymes involved in L-cysteine biosynthesis of Entamoeba histolytica.

Serine acetyltransferase (SAT; EC 2.3.1.30) catalyzes the CoA-dependent acetylation of the side chain hydroxyl group of l-serine to form O-acetyl serine, in the first step of the L-cysteine biosynthetic pathway. Since this pathway is selectively present in a few parasitic protists and absent in mammals, it represents a reasonable target to develop new chemotherapeutics. Entamoeba histolytica apparently possesses three SAT isotypes (EhSAT1-3) showing 48-73% mutual identity, a calculated molecular mass of 34.4-37.7 kDa, and an isoelectric point of 5.70-6.63. To better understand the role of individual SAT isotypes, we determined kinetic and inhibitory parameters of recombinant SAT isotypes. While the three SAT isotypes showed comparable Km and k(cat) for L-serine and acetyl-CoA, they showed remarkable differences in their sensitivity to inhibition by L-cysteine. The Ki values for L-cysteine varied by 100-fold (4.7-460 microM) among SAT isotypes (EhSAT1