Defining functional domains within PF16: a central apparatus component required for flagellar motility

Mutations affecting the assembly and stability of the central apparatus result in flagellar paralysis. Chlamydomonas cells with mutations at the PF16 locus have paralyzed flagella, and the C1 microtubule of the central apparatus is missing in isolated axonemes. On the basis of its mutant phenotype, sequence, and localization, PF16, a member of the armadillo repeat containing family of proteins, is involved in protein-protein interactions required for stability of the C1 microtubule and flagellar motility. Previous biochemical analysis of flagella isolated from pf16 cells demonstrated that assembly of the PF16 protein is either dependent on, or required for, the assembly of at least two other flagellar components. As a first step toward identifying functional domains in the PF16 protein that are essential for these interactions, we have characterized three mutations at the PF16 locus. In addition, we have generated deletion constructs of the PF16 gene and tested for their ability to assemble and rescue motility upon transformation of mutant pf16 cells. Our results demonstrate that the first armadillo repeat is necessary but not sufficient for assembly; that the C-122 amino acids are not required for assembly or motility; and that the repeats appear to form a single functional unit required for PF16 assembly.