The deduced amino acid sequence of the open reading frame corresponds to a protein containing 144 amino acids, indicating that PfI2 has the shortest amino acid sequence among I2 homologs. Sequence alignment com bined with visual inspection obviously of PfI2 showed an overall identity of 28% and 34% identity between amino acids at positions 5 to 105 of PfI2 when compared to human I2. The use of PSORTII software revealed a putative nuclear localization signal. The PfI2 sequence, found in human I2 and shown to be required protein contains two peptides KTISW and KHYNE that fit perfectly to the or RV F motif and HYNE motifs responsible for binding to PP1c. However, 2 main differences were observed for interaction with PP1c is not present in the PfI2 sequence and second, the KSQKW sequence of human I2 contains a Q residue instead of V or I of the RV F consensus sequence.

The analysis of PfI2 using protein secondary structure prediction soft ware PsiPred predicted that the RV F motif is a part of an unstructured region, while the HYNE motif is within an heli occurring between positions 70 and 120. This structure is in agreement with that identified in mammalian I2. This analysis is in accordance with the structure prediction presented in PlasmoDB. A ma imum likehood phylogenetic tree was generated under the JTT I G model with the support of two outgroups composed of two well described PP1 regula tors Inhibitor 3 and LRR1. In this tree PfI2 segregates with orthologues from other Plasmodium species as well as the apicomple an Theileria parva, but within the I2 family on a well supported branch separate from the I3 family.

This analysis clearly identi fies PfI2 as a PP1c inhibitor 2 family member. E pression of PfI2 protein by P. falciparum and localization studies To investigate the e pression of PfI2 by P. falciparum, GSK-3 polyclonal antibodies against the recombinant PfI2 protein were raised. As presented in Figure 2A lane 1, the recombinant protein whose amino acid sequence was confirmed by MALDI TOF mass spectrometry, migrated at about 20 kDa, in agree ment with the anomalous electrophoretic behavior of inhibitors of the PP1 family. the e pected molecular weight of endogenous PfI2 is 16. 7 KDa. Although these antibodies recognized the recombinant protein, they were unable to react with any bands in total e tracts of asynchronous blood stage parasites. In order to detect endogenous PfI2, we carried out immunoprecipita tion e periments with anti PfI2 sera or pre bleed sera with total parasite e tracts. Immunoblots with anti PfI2 anti bodies showed the presence of a band at 20 kDa in the immunoprecipitates with anti PfI2, while the pre bleed serum detected no specific band.