A bipartite Ca2+-regulated nucleoside-diphosphate kinase system within the Chlamydomonas flagellum -: The regulatory subunit p72

Regulation of flagellar activity in Chlamydomonas involves both Ca2+ and cAMP-mediated signaling pathways. However, Chlamydomonas and sea urchin sperm flagella also exhibit nucleoside-diphosphate kinase (NDK) activity, suggesting a requirement for GTP within this highly conserved organelle. In sea urchin sperm, the NDK catalytic subunit is an integral component of the outer dynein arm. Here we describe a modular protein (p72) from the Chlamydomonas flagellum that consists of three domains closely related to the presumptive regulatory segment of rat NDK-7 followed by two EF-hands that are predicted to bind Ca2+. There are close homologues of p72 in both mammalian and insect genomes. The p72 protein is tightly associated with the flagellar axoneme and is located along the entire length except at the transition zone. Cross-linking experiments suggest that p72 interacts with two or three additional axonemal polypeptides. The sensitivity of p72 to tryptic digestion differed considerably in the presence and the absence of Ca2+, suggesting that it indeed binds this ligand. These studies indicate that the flagellar NDK system is bipartite with the regulatory and catalytic components residing on different polypeptides. We propose that Ca2+ regulation of flagellar motility in Chlamydomonas may be achieved in part through a downstream GTP-mediated signaling pathway.