Adam Zwolak

A vertebrate myosin-I structure reveals unique insights into myosin mechanochemical tuning.

Significance We report the high-resolution structure of a tension-sensing myosin-Ib. We identify a striking unique orientation of structural elements that position the motor’s lever arm. This orientation results in a cavity between the motor and lever arm that holds a 10-residue stretch of N-terminal amino acids, a region that is divergent among myosins. We show the importance of the N-terminal region of myosin in controlling the kinetics and mechanics of the motor. Myosins are molecular motors that power diverse cellular processes, such as rapid organelle transport, muscle contraction, and tension-sensitive anchoring. The structural adaptations in the motor that allow for this functional diversity are not known, due, in part, to the lack of high-resolution structures of highly tension-sensitive myosins. We determined a 2.3-Å resolution structure of apo-myosin-Ib (Myo1b), which is the most tension-sensitive myosin characterized. We identified a striking unique orientation of structural elements that position the motor’s lever arm. This orientation results in a cavity between the motor and lever arm that holds a 10-residue stretch of N-terminal amino acids, a region that is divergent among myosins. Single-molecule and biochemical analyses show that the N terminus plays an important role in stabilizing the post power-stroke conformation of Myo1b and in tuning the rate of the force-sensitive transition. We propose that this region plays a general role in tuning the mechanochemical properties of myosins.

PICK1 is implicated in organelle motility in an Arp2/3 complex–independent manner

A SAXS-based structural model is described for PICK1, a key player in AMPA receptor trafficking. It is shown that the acidic C-terminal tail of PICK1 is involved in autoinhibition and motility of PICK1-associated vesicle-like structures, but, contrary to previous reports, PICK1 neither binds nor inhibits Arp2/3 complex.

CARMIL leading edge localization depends on a non-canonical PH domain and dimerization

CARMIL is a ~1370 amino acid cytoskeletal scaffold that plays crucial roles in cell motility and tissue development through interactions with cytoskeletal effectors and regulation of capping protein at the leading edge. However, the mechanism of CARMIL leading edge localization is unknown. Here we show that CARMIL interacts directly with the plasma membrane through its N-terminal region. The crystal structure of CARMIL1-668 reveals that this region harbors a non-canonical pleckstrin homology (PH) domain connected to a 16 leucine-rich repeat domain. Lipid binding is mediated by the PH domain, but is further enhanced by a central helical domain. Small angle x-ray scattering reveals that the helical domain mediates antiparallel dimerization, properly positioning the PH domains for simultaneous membrane interaction. In cells, deletion of the PH domain impairs leading edge localization. The results support a direct membrane binding mechanism for CARMIL localization at the leading edge, where it regulates cytoskeletal effectors and motility.