Institute for Cell Biology, University of Munich, Germany

Unexpected links between cellular organelles continue to unfold.

As a graduate student, I was intrigued by centrioles. Their beautiful ninefold symmetry, occurrence in pairs and positioning at the heart of the cell —as a constituent of the microtubule-organizing centrosome — made them appear significant. But how they are built and replicated, and even their true purpose in cell division, remained enigmatic.

Adding to my fascination was the fact that, in my cultured cells, one of the centrioles often acted as an anchor, or basal body, for a cilium-like appendage — even though the endothelial cells I used were not thought to ever have cilia.

Once considered a biological oddity, these stubby, non-motile 'primary' cilia are now known to be present in most cells of the human body (see here) and probably serve as essential sensors whose disturbance is linked to a growing list of diseases.

I have always wondered whether there is an additional functional link between centrioles and primary cilia. A recent paper (A. Robert et al. J. Cell Sci. 120, 628–637; 2007) shows that a protein involved in the biogenesis of primary cilia is also a bona fide constituent of the centrosome. Moreover, this centrosomal/ciliary component is tied into the regulation of the cell cycle.

This finding joins a series of recent discoveries of proteins that are both centrosomal and ciliary. Taken together, these studies are revealing a truly novel functional link: centrioles are there to make primary cilia; primary cilia act as sensors of external stimuli; and, as other studies have shown, external stimuli can regulate cell proliferation. So the mysterious organelles of my graduate student days are being demystified, but the story that unfolds turns out to be even more fascinating than I expected.