Changes in the Adenylate Kinase Activity are Proportional to the ADP/ATP Ratio Upon Resorption and Regeneration of Chlamydomonas reinhardtii Flagella.

Adenylate kinases (ADK) maintain cellular energy homeostasis and catalyse a reversible reaction that converts two molecules of ADP into ATP and AMP. ATP in Chlamydomonas reinhardtii flagella is utilised by dynein to generate flagellar beating. ATP must be constantly supplied and maintained; however, the constricted nature of flagella restricts the localisation of mitochondria in vicinity. We show that C. reinhardtii flagella carry conserved ADK domain-containing proteins that are large in number and longer than their cytosolic counterparts. Six of the eight flagellar ADKs are enriched in the central pair apparatus (CPA). Upon flagellar regeneration and resorption, the ADK activity changes, suggesting a shift in the energy demands for the two processes. The total ADK activity in regenerating flagella increased, and resorbing flagella showed an equal but reverse effect. ADKs help regenerate ATP locally and act as phosphotransfer agents that spatially direct the transfer of nucleotides. The ADP to ATP ratio during reflagellation and resorption suggests a role for ADKs in maintaining the nucleotide levels. To the best of our knowledge, this is the first study providing evidence for the role of ADK domain-containing proteins in maintaining ATP homeostasis in response to flagella regeneration and resorption.