Initiation of translation and cellular localization of Theileria annulata casein kinase IIα: Implication for its role in host cell transformation

Theileria annulata and T. parva are protozoa that infect bovine leukocytes which leads to subsequent transformation and uncontrolled proliferation of these cells. It has been proposed that the CKIIα subunit of T. parva induces mitogenic pathways of host leukocytes by being exported into the host cell. The evidence for this is the existence of a predicted N‐terminal secretion signal‐like peptide. We tested this hypothesis by analyzing gene structure, translation, and protein localization of the T. annulata CKIIα (TaCKIIα). The determined TaCKIIα‐ORF potentially codes for a 50 kDa protein with an N‐terminal extension including a possible signal sequence not present in CKIIα proteins of non‐Theileria species. However, antisera raised against TaCKIIα recognized a protein of a molecular weight of about 40 kDa and, therefore, inconsistent with this predicted molecular weight. We demonstrate by in vitro transcription/translation that this discrepancy is due to translation from a downstream initiation site omitting the putative N‐terminal signal sequence and thus excluding the notion that the protein product is secreted via the classical secretory pathway. In corroboration immunofluorescence investigations suggest that the TaCKIIα subunit is confined to the parasite schizonts within the host cell. On the basis of the above findings it seems highly unlikely that export via the classical pathway of the parasite CKIIα is the way in which this protein possibly contributes to host cell transformation. J. Cell. Physiol. 196: 444–453, 2003. © 2003 Wiley‐Liss, Inc.