We are interested in cell motility and muscle contraction, with particular focus on the function and regulation of myosin superfamily. Myosin is a type of mechanoenzyme, which converts energy from ATP hydrolysis into mechanic movements, and participates a number of important cellular processes including cell movement, intracellular trafficking, signaling transduction, muscle contraction etc.
1. Structure and function of unconventional myosin
Besides being a key component of muscle, myosins exist in all types of eukaryotic cells. With the completion of a number of eukaryotic genomic projects, it became apparent that the myosin superfamily is much larger and more diverse than previously predicted. Currently, the myosin superfamily is organized in more than 35 classes, based on structural similarities. Although the functions of conventional myosins in muscle contraction and cell division have been intensively studied, little is known about the unconventional myosin. We are currently investigating two unique unconventional myosins, myosin-19 and -20.
Myosin-19 is a vertebrate-specific myosin and is exclusively localized in mitochondria. Recently, we found that myosin-19 is a high-duty ratio molecular motor moving to the plus-end of the actin filament (JBC 2014). We are currently investigating the role of myosin-19 on mitochondria biology. Myosin-20 is an insect-specific myosin. Drosophila myosin-20 functions as a crucial downstream component of the Fat signaling pathway, influencing growth, affinity, and gene expression during development. We have recently shown that Drosophila myosin-20 functions as a scaffold protein, but not as a molecular motor, in a signaling pathway controlling cell differentiation (Biochem 2014).
2. Molecular regulation of unconventional myosin-5.
Myosin-5 (Myo5) is so far the best characterized unconventional myosin in term of cellular function and molecular regulation. Myo5 is widely expressed in almost all eukaryotes and implicated in a number of vesicles trafficking and tethering. We are particularly interested in how the motor function of Myo5 is regulated. We proposed a tail-inhibition model for the regulation of vertebrate Myo5a: The globular tail domain (GTD) of Myo5a is the inhibitory domain, which interacts with the head and inhibit its motor function; cargo-binding, high Ca2+ levels and/or phosphorylation may reduce the head-GTD interaction, thus activating the motor activity (BBRC 2004, JBC 2005,2006, PNAS 2008). This model for the regulation of Myo5 is well accepted in the field and becomes a paradigm for the regulation of unconventional myosin.
We found that vertebrate Myo5b and Myo5c, as well as Drosophila Myo5 is regulated by the GTD in a Ca2+-dependent manner (Cell 2008, JBC 2016a, Biochem J 2015), suggesting that tail-inhibition mechanism is conserved for class V myosins. We demonstrated that the GTD functions as a dimer in inhibiting the motor function of the head (JBC 2016b) and proposed that cargo-binding allosterically abolishes the head-GTD interaction, thus activating the motor function (Sci Rep 2015). We identified the calmodulin bound to the first IQ motif of Myo5a as the key for Ca2+ activation (JBC 2012). We solved the structure of myosin-5a in complex with Ca2+-bound calmodulin and characterized Ca2+ transition in the complex, and proposed that the calmodulin in complex with IQ motif functions as an intact Ca2+ sensor (PNAS 2016).
3. Molecular mechanism of insect muscle development
The migratory Locusts (Locusta migratoria) are important agricultural pests in China. The plague of Locusts is associated with the strong ability in moving and flying, which depends on locust muscle system. Striated muscle myosin is the most important functional protein in muscle. We found that, contrary to vertebrates, locust has only one striated muscle myosin gene, containing six alternative splicing exons in Locust myosin gene. In theory, locust myosin gene encodes 360 types of myosin (IMB 2016). We are currently investigating how the alternative splicing exons affect the function of Locust muscle myosin and how the alternative splicing exons of Locust myosin is regulated.
