Characterizing interactions between the microtubule-binding protein CLIP-170 and F-actin

The cooperation between the actin and microtubule (MT) cytoskeletons is important for cellular processes such as cell migration and muscle cell development. Full understanding of how this cooperation occurs has yet to be sufficiently developed. The MT plus-end tracking protein (+TIP) CLIP-170 has been implicated in this actin-MT coordination by associating with the actin-binding signaling protein IQGAP1, and by promoting actin polymerization through binding with formins. Thus far, CLIP-170s interactions with actin were assumed to be indirect. Here, we demonstrate that CLIP-170 can bind to filamentous actin (F-actin) directly. The affinity is relatively weak, but is strong enough to be significant in the actin-rich cortex, where actin concentrations can be extremely high. Using CLIP-170 fragments and mutants, we show that the direct CLIP-170:actin interaction is independent of the FEED domain, the region that mediates formin-dependent actin polymerization, and that the CLIP-170 F-actin-binding region overlaps with the MT-binding region. Consistent with these observations, in vitro competition assays indicate that CLIP-170:F-actin and CLIP-170:MT interactions are mutually exclusive. Taken together, these observations lead us to speculate that direct CLIP-170:F-actin interactions may function to reduce the stability of MTs in actin-rich regions of the cell, as previously proposed for EB1.