The identification and initial characterization of the ApiAP2 family of transcriptional regulators is a major step toward understanding gene regulation in Plasmodium spp.
It has been established that the majority of Plasmodium
ApiAP2 family members interact with sequence specific elements and have the potential to function as trans-
acting factors [42
]. Experiments on individual ApiAP2 factors, in both the mosquito and blood stages of development, have begun to answer some fundamental questions regarding the in vivo
function of this protein family [37
]. However, the exact role that each ApiAP2 protein is playing in the biology of the parasite largely remains to be determined, emphasizing the necessity for more in vivo
Although the 60 amino acid AP2 domains are highly conserved among the Apicomplexa
, the sequence similarity does not extend to the rest of the protein and homology outside of the AP2 DNA-binding domain is low [22
]. With the wide range of sizes predicted for ApiAP2 proteins, it can be anticipated that there are additional domains that activate transcription or promote interaction with other proteins required for regulation. Furthermore, there is evidence to suggest that some ApiAP2 proteins may be processed during parasite development implying that multiple forms may exist for individual proteins [37
]. PFF0200c (PfSIP2) has been shown to be proteolytically processed from the full length 230 kDa form to a 50–60 kDa N-terminal segment containing the two AP2 DNA-binding domains [37
]. Regarding other active regions outside of the AP2 domains, a portion of PFF0200c (amino acids 177–313) has been demonstrated to act as a transactivation domain in a yeast system and has subsequently been exploited in a series of tetracycline transactivator-based inducible conditional knockout vectors for P. berghei
], P. Pino, E. Bush, O. Billker, M. Llinás, D. Soldati, unpublished data).
Beyond these experiments, it is unknown what other domains may exist and how they participate in coordinating repression or activation of target genes. Furthermore, the identification of specific interacting partners of ApiAP2 proteins will be a major advance in understanding in vivo functions for many of these factors. As shown by the work summarized herein, specific ApiAP2 proteins can directly regulate subsets of genes involved in Plasmodium developmental transitions or promote the formation of heterochromatin in subtelomeric regions. Although the majority of AP2 domains bind DNA in vitro, it is currently unknown how the DNA-tethered ApiAP2 proteins mediate interactions with proteins of the general transcription complexes or chromatin remodeling machinery. Demonstration of protein-protein interactions, such as with RNA polymerase II or GCN5, a heterochromatin remodeler, would further support the role of ApiAP2 proteins in transcriptional regulation.
In addition to the specific functional questions for individual ApiAP2 factors lie broad questions regarding the family as a whole. Foremost is the issue of whether the ApiAP2 family will perform parallel functions in all Apicomplexa
, or whether there is species-specific variability. Given the virtually complete sequence identity of individual AP2 DNA-binding domains across the Apicomplexa
, it is likely that the same DNA sequence elements will be bound in different organisms, however, target genes under the control of these DNA motifs may vary greatly from species to species [28
]. Preliminary evidence shows that the AP2-O motif (TAGCTA) is conserved upstream of common target genes in P. berghei
, P. vivax,
and P. falciparum
], but other motifs share little overlap in target gene predictions between different Plasmodium
]. Dissecting such differences in target regulons will ultimately illuminate the species-specific regulatory functions served by this protein family.
In conclusion, the ApiA2 proteins are excellent candidate regulators of the precisely timed and coordinated gene expression governing multi-stage development and inter-host transitions required for the successful propagation of Plasmodium parasites and the other Apicomplexa. We anticipate that a better understanding of these factors will facilitate the exploration of new therapeutic interventions in the foreseeable future. Such strategies might be aimed at disrupting the interaction of ApiAP2 proteins and their target DNAs or interfering with protein-protein interactions crucial for their downstream function, with the ultimate goal of arresting parasite development. The in-depth characterization of ApiAP2 proteins will fill a void in apicomplexan biology that is crucial to fully understanding parasite development and pathogenesis and may reveal mechanisms underlying host selection and disease severity.
- The ApiAP2 protein family are candidate transcription regulators in Plasmodium spp.
- ApiAP2 proteins play a role in gene regulation at various stages of Plasmodium development
- Most of the P. falciparum ApiAP2 have been shown to bind sequence specific DNA elements