Arf GTPase-activating protein ASAP1 specifically binds to 440-kD ankyrin-B

The 440-kD giant ankyrin-B (gAnkB) exclusively localizes to axons and is essential for axon development. However, proteins that specifically bind to gAnkB but not to other isoforms of ankyrins are poorly understood. Here, we discovered that an Arf GTPase-activating protein ASAP1 and ASAP2 specifically binds to a short and disordered sequence unique to gAnkB. Biochemical studies showed that the SH3 domain of ASAP1 binds to a 12-residue, positively charged peptide fragment from gAnkB. The high-resolution structure of the ASAP1-SH3 domain in complex with the gAnkB peptide revealed the mechanism underlying this non-canonical SH3 domain-mediated target recognition. Further structural and bioinformatic analysis revealed additional previously unknown ASAP1-SH3 binding partners including Clasp1 and Clasp2, both of which are well-known microtubule regulators. Among all known ASAP1-SH3 binders including those identified in the current study, gAnkB has the strongest affinity in binding to ASAP1. Our results suggest that ASAP1 may function together with gAnkB in regulating axonal cytoskeletons.