Filamin: a scaffolding protein

Filamin is an actin-binding protein known to be essential for wild type slug phototaxis in D. discoideum. The protein contains an actin-binding domain (ABD) followed by a rod domain containing 6 closely related repeat segments. Here I show that repeats 2-6 in the rod domain and the ABD are essential for wild type phototaxis, suggesting that when these segments are missing proteins which bind to these segments are excluded from the photosensory pathway and phototaxis is impaired. Repeat 1 was shown not to be essential if filamin1 was overexpressed to levels 10X higher than wild type filamin. This indicates that repeat 1 is not involved in directly binding proteins but is still required for the formation of fully functional complexes. Coimmunoprecipitation experiments identified 5 filamin-interacting proteins-RasD, Erkb, AMPK, PKB & GRP125. The interaction with RasD did not rely on a single repeat in the rod domain nor the ABD. It is unknown how filamin interacts with the other proteins involved in phototaxis but it''s probable that filamin acts as a scaffold for the assembly of a photosensory signalling complex through its repeats, predominantly repeats 2-6.