The EFA6 (Exchange Factor for Arf6) family is conserved from yeast to human, originally named after the Sec7 homology domain that has GEF activity specific to ARF6 GTPases. Functional studies have shown that EFA6 family proteins can also regulate neuronal microtubule (MT) dynamics, involving a conserved 18 aa motif embedded in an intrinsic disordered region in the EFA6 N-terminus (O'rourke et al., Nat. Cell Biol., 2010). EFA6 proteins additionally contain a pleckstrin homology (PH) domain and a coiled-coil domain that enable their association with plasma membrane and actin cytoskeleton. In C. elegans, both expression level and localization of EFA-6 are critical for its function in neurons (Chen et al., Neuron, 2011; Chen et al., Elife, 2015). However, endogenous expression of EFA-6 remains unknown. To dissect mechanisms regulating EFA-6, we first generated a GFP knock-in to tag endogenous EFA-6. Wild-type GFP::EFA-6 is found in multiple tissues, including neurons, pharynx, epidermal cells and germline. We then carried out a visual genetic screen and isolated several classes of mutants that affect GFP::EFA-6 localization in various tissues. Among these we focused on the ones showing EFA-6 mis-localization in touch receptor neurons (TRNs) or adjacent epidermal cells. Combining whole-genome sequencing and genetic mapping, we identified several components of the specialized extracellular matrix (ECM) for TRNs, known as the mantle, which has been well studied for its role in touch sensitivity. The mantle consists of MEC-1, MEC-5, MEC-9, and HIM-4 (Emtage et al., Neuron, 2004). MEC-1 is essential for accumulation of the collagen MEC-5 and other ECM components. MEC-5 and MEC-9 do not affect the general structure of mantle and attachment of the neurons to the epithelia by themselves. The hemicentin HIM-4 functions separately from the MEC ECM proteins, and is needed for TRN attachment but not touch sensitivity. Our results suggest that
mec-1,
mec-5, and
him-4 have similar effects on EFA-6 localization and may act in a common pathway. We also find that disruption of the TRN specific tubulins MEC-7 and MEC-12 affect EFA-6 localization. We are currently investigating how ECM interacts with MTs to regulate EFA-6 localization and function in neurons and other tissues.