Differential Localization of the Two T. brucei Poly(A) Binding Proteins to the Nucleus and RNP Granules Suggests Binding to Distinct mRNA Pools

The number of paralogs of proteins involved in translation initiation is larger in trypanosomes than in yeasts or many metazoan and includes two poly(A) binding proteins, PABP1 and PABP2, and four eIF4E variants. In many cases, the paralogs are individually essential and are thus unlikely to have redundant functions although, as yet, distinct functions of different isoforms have not been determined. Here, trypanosome PABP1 and PABP2 have been further characterised. PABP1 and PABP2 diverged subsequent to the differentiation of the Kinetoplastae lineage, supporting the existence of specific aspects of translation initiation regulation. PABP1 and PABP2 exhibit major differences in intracellular localization and distribution on polysome fractionation under various conditions that interfere with mRNA metabolism. Most striking are differences in localization to the four known types of inducible RNP granules. Moreover, only PABP2 but not PABP1 can accumulate in the nucleus. Taken together, these observations indicate that PABP1 and PABP2 likely associate with distinct populations of mRNAs. The differences in localization to inducible RNP granules also apply to paralogs of components of the eIF4F complex: eIF4E1 showed similar localization pattern to PABP2, whereas the localisation of eIF4E4 and eIF4G3 resembled that of PABP1. The grouping of translation initiation as either colocalizing with PABP1 or with PABP2 can be used to complement interaction studies to further define the translation initiation complexes in kinetoplastids.