UCSF

How the complex spatial organization of a eukaryotic cell is achieved is an enduring and fundamental question in cell biology. The size and arrangement of organelles within the cell factor significantly in the establishment of cell architecture, yet little is known about how organelle size is controlled. This question is most easily addressed through study of linear structures, for which there is only one dimension that must be quantified. We have utilized the eukaryotic flagellum, a microtubule-based structure that protrudes outside the cell, as a model organelle to investigate the issue of organelle size control.

Eukaryotic flagella, alternatively known as cilia, are also known to be crucial for many developmental and physiological processes, yet their biogenesis, maintenance, and length control are not well understood. We have taken a genetic approach to understanding the system of flagellar length control in the unicellular green alga Chlamydomonas reinhardtii. Chlamydomonas cells possess two flagella, each typically 10-12μm in length, and this length is rigorously maintained. Whereas previous analyses of length control have concentrated upon long flagella (lf) mutants, we have thoroughly examined a novel collection of twenty short flagella (shf) mutants recovered from an insertional mutagenesis screen.

We have analyzed the flagellar length distributions, regeneration kinetics, and gene expression of these shf strains, and identified a class of mutants with defects in regeneration of the flagellar precursor pool. Of these precursor pool mutants, one appears to have a defect in flagellar gene induction during regeneration, and the others appear to have post-transcriptional defects. We have identified the molecular lesion of one of the strains with a post-transcriptional defect, and in doing so uncovered a previously unknown role for the microtubule-severing heterodimer katanin in flagellar length control.