Kinesin II Hauls Signaling Cargo

Abstract

The unicellular green alga Chlamydomonas uses a flagellum not only for locomotion, but also for presentation of flagellar adhesion molecules that promote interaction between gamete cells and initiate signaling events important to fertilization. The flagella lack the apparatus to make new proteins, so the cells use an intraflagellar transport system to transport particles containing proteins made in the cell body to the flagella. Such transport occurs constitutively, but regulated transport has also been described. For example, the Chlamydomonas aurora protein kinase (CALK) is transported to the flagellum only in response to signals generated when receptors on flagella of cells of opposite mating types interact. (Interestingly, aurora protein kinases are implicated in cell division in other systems.) Pan and Snell used mutants of the motor protein kinesin II to show that kinesin II is required for such activated transport of CALK to the flagellum. Kinesin II is also the motor responsible for the constitutive transport of proteins to flagella. However, effects of the kinesin II mutation in cells maintained at different temperatures could distinguish between the two roles of kinesin II, and the regulated transport of CALK appeared to result in association with the flagellar membrane whereas unregulated transport delivers proteins to a soluble cytoplasmic location. Schnapp reviews other evidence for active transport of signaling molecules, which in some cases involves interaction of motor proteins with scaffold proteins, allowing whole signaling complexes to be delivered to the appropriate site of action.J. Pan, W. J. Snell, Kinesin II and regulated intraflagellar transport of Chlamydomonas aurora protein kinase. J. Cell Sci. 116, 2179-2186 (2003). [Abstract] [Full Text]B. J. Schnapp, Trafficking of signaling modules by kinesin motors. J. Cell Sci. 116, 2125-2135 (2003). [Abstract] [Full Text]