Flagella and cilia serve as molecular highways. Sedmak and Wolfrum have for the first time tracked down several key proteins involved in this transport. They also discovered the proteins in cells that have neither cilia nor flagella, indicating that the molecules have additional functions.
Traffic in cilia and flagella runs in both directions. Kinesin motors haul cargo up, while dynein molecules ferry it down. Crucial for this movement are intraflagellar transport (IFT) proteins, which researchers think cluster into complexes. However, where the individual IFT proteins settle in the cell and what they do are unclear.
Using immunoelectron microscopy, Sedmak and Wolfrum pinpointed five IFT proteins in photoreceptor cells from the retina. The photoreceptor cell's outer segment harbors light-sensitive pigments. All the active organelles reside in the inner segment of the cell. The connecting cilium is the only cytoplasmic bridge through which cargoes can pass between the segments.
The researchers found that the five IFT proteins didn't always occur together, suggesting that they perform different tasks during intraflagellar transport. For example, at the base of the connecting cilium, cargoes leave the microtubules that transported them through the inner segment and switch to the cilium for the trip to the outer segment. Three of the IFT proteins clustered at this transfer station, slightly apart from the other two. This separation might indicate that the two protein bunches load different cargoes onto the cilium. Another difference involves the protein IFT20, the only one that appeared in the Golgi apparatus. Its job could include sorting molecules destined for the cilium.
To the researchers' surprise, when they checked the dendrites of neurons that don't carry cilia or flagella, they also spotted IFT proteins on cargo vesicles. Last year, a study found the proteins in T cells, which also lack the structures. These findings broaden the range of cells that rely on IFT proteins and suggest that they also take part in non-ciliary transportation.