Regulated targeting of a protein kinase into an intact flagellum - An aurora/Ipl1p-like protein kinase translocates from the cell body into the flagella during gamete activation in Chlamydomonas

In the green alga Chlamydomonas reinhardtii flagellar adhesion between game tes of opposite mating types leads to rapid cellular changes, events collec tively termed gamete activation, that prepare the gametes for cell-cell fus ion, As is true for gametes of most organisms, the cellular and molecular m echanisms that underlie gamete activation are poorly understood. Here we re port on the regulated movement of a newly identified protein kinase, Chlamy domonas aurora/Ipl1p-like protein kinase (CALK), from the cell body to the flagella during gamete activation. CALK encodes a protein of 769 amino acid s and is the newest member of the aurora/Ipl1p protein kinase family. Immun oblotting with an anti-CALK antibody showed that CALK was present as a 78/8 0-kDa doublet in vegetative cells and unactivated gametes of both mating ty pes and was localized primarily in cell bodies. In cells undergoing fertili zation, the 78-kDa CALK was rapidly targeted to the flagella, and within 5 min after mixing gametes of opposite mating types, the level of CALK in the flagella began to approach levels normally found in the cell body, Protein synthesis was not required for targeting, indicating that the translocated CALK and the cellular molecules required for its movement are present in u nactivated gametes. CALK was also translocated to the flagella during flage llar adhesion of nonfusing mutant gametes, demonstrating that cell fusion w as not required for movement. Finally, the requirement for flagellar adhesi on could be bypassed; incubation of cells of a single mating type in dibuty ryl cAMP led to CALK translocation to flagella in gametes but not vegetativ e cells. These experiments document a new event in gamete activation in Chl amydomonas and reveal the existence of a mechanism for regulated translocat ion of molecules into an intact flagellum.