Role of arginine kinase in Paramecium tetraurelia (Ciliophora, Peniculida): Subcellular localization of AK3 and phosphoarginine shuttle system in cilia.

The ciliate Paramecium tetraurelia has four arginine kinase genes (AK1, AK2, AK3, and AK4). Of these genes, only AK3 has a signal sequence for farnesylation, a post-translational modification that enables anchoring of the modified enzyme to the ciliary membrane. To confirm this modification, AK3 was synthesized using a cell-free protein synthesis system and the peptide masses were analyzed using peptide mass fingerprinting (PMF). The PMF analysis indicated that the C-terminal peptide of AK3 is farnesylated. Thus, AK3 can be farnesylated under physiologically appropriate conditions. To determine the subcellular localization of P. tetraurelia AK3, Western blot analysis was performed using an AK3 polyclonal antibody for the proteins extracted from intact cells and ciliary fractions. When extraction was performed using Triton X-100, AK3 was detected the ciliary fraction. This result suggested that the ciliary fraction contains AK3. In addition, we investigated the role of P. tetraurelia AKs in ciliary movement using the feeding RNA interference method. The swimming velocity of AK1- and AK3-silenced cells was significantly reduced to half the value of that control cells. In summary, P. tetraurelia AK3 is likely to be located in the ciliary membrane and influences swimming velocity, presumably through the phosphoarginine shuttle system present in cilia.