© 2019. Published by The Company of Biologists Ltd | Journal of Cell Science (2019) 132, jcs227132. doi:10.1242/jcs.227132 CELL SCIENCE AT A GLANCE Dynein activators and adaptors at a glance Mara A. Olenick and Erika L. F. Holzbaur* ABSTRACT ribonucleoprotein particles for BICD2, and signaling endosomes for Cytoplasmic dynein-1 (hereafter dynein) is an essential cellular motor Hook1. In this Cell Science at a Glance article and accompanying that drives the movement of diverse cargos along the microtubule poster, we highlight the conserved structural features found in dynein cytoskeleton, including organelles, vesicles and RNAs. A long- activators, the effects of these activators on biophysical parameters, standing question is how a single form of dynein can be adapted to a such as motor velocity and stall force, and the specific intracellular wide range of cellular functions in both interphase and mitosis. functions they mediate. – Recent progress has provided new insights dynein interacts with a KEY WORDS: BICD2, Cytoplasmic dynein, Dynactin, Hook1, group of activating adaptors that provide cargo-specific and/or Microtubule motors, Trafficking function-specific regulation of the motor complex. Activating adaptors such as BICD2 and Hook1 enhance the stability of the Introduction complex that dynein forms with its required activator dynactin, leading Microtubule-based transport is vital to cellular development and to highly processive motility toward the microtubule minus end. survival. Microtubules provide a polarized highway to facilitate Furthermore, activating adaptors mediate specific interactions of the active transport by the molecular motors dynein and kinesin. While motor complex with cargos such as Rab6-positive vesicles or many types of kinesins drive transport toward microtubule plus- ends, there is only one major form of dynein, cytoplasmic dynein-1, University of Pennsylvania Perelman School of Medicine, Philadelphia, PA 19104, which drives the trafficking of a wide array of minus-end-directed USA. cargo within the cell. Recent work has brought new insight into the spatial and temporal regulation of cytoplasmic dynein by adaptor *Author for correspondence ([email protected]) proteins, which link dynein to cargo (Fu and Holzbaur, 2014; E.L.F.H., 0000-0001-5389-4114 Kardon and Vale, 2009; Reck-Peterson et al., 2018). Interestingly, Journal of Cell Science 1 CELL SCIENCE AT A GLANCE Journal of Cell Science (2019) 132, jcs227132. doi:10.1242/jcs.227132 many of these adaptors modulate the motile properties of dynein, heads of the dynein dimer are reoriented to facilitate motility either enhancing or inhibiting movement, while some act as motility (Zhang et al., 2017); single-molecule studies have shown that the switches by co-ordinately regulating dynein and kinesin. Here, we complex between dynein–dynactin and an activating adaptor is use ‘adaptor’ as a more general term to discuss proteins that can link superprocessive compared to the motility of dynein alone dynein to cargo and use ‘activating adaptor’ or ‘activator’ to identify (McKenney et al., 2014; Schlager et al., 2014a), moving at a subclass of adaptor proteins that have been shown to enhance the velocities ranging from ∼0.4 to 1.4 µm/s over run-lengths of up to processivity of dynein. Below, and in the accompanying poster, we 40 µm. Furthermore, some activating adaptors such as Hook3 can summarize our current understanding of both the structure and recruit two dynein dimers to one dynactin complex, enhancing both function of dynein adaptor proteins. the velocity and the force produced by the motor complex (Urnavicius et al., 2018; Grotjahn et al., 2018). While it is Dynein and dynactin difficult to compare velocity and run length data across multiple Cytoplasmic dynein 1 (henceforth referred to as dynein) is a 1.4 studies due to differing experimental conditions such as buffer MDa motor complex consisting of dimerized heavy chains (DHCs; composition and ionic strength, studies that have directly compared symbol DYNC1H1), each with an N-terminal tail and a C-terminal dynein activators have found differences in velocities and force motor domain (see poster). The N-terminal tail mediates production that suggest that these cofactors can fine-tune dynein homodimerization of the heavy chains, along with binding sites motor function (McKenney et al., 2014; Olenick et al., 2016; for non-catalytic subunits, including two intermediate chains (DICs; Redwine et al., 2017; Urnavicius et al., 2018). DYNC1I1 and DYNC1I2) and two light intermediate chains (LIC1 and LIC2, also known as DYNC1LI1 and DYNC1LI2). Dynein activators Additional light chains (LCs) are also bound to the DICs. These BICD proteins were initially identified in Drosophila where non-catalytic subunits of dynein are thought to stabilize the mutations cause abnormal development of the abdomen resulting complex and may contribute to the regulation of specific dynein in a bicaudal (‘two-tailed’) phenotype (Mohler and Wieschaus, functions (recently reviewed by Reck-Peterson et al., 2018). The 1986). Drosophila BICD was found to be vital for mRNA transport motor domain of dynein is composed of six concatenated AAA+ in ribonucleoprotein (RNP) complexes and nuclear positioning domains that form a motor ring with a protruding flexible 15-nm- (Wharton and Struhl, 1989; Suter and Steward, 1991; Swan and long stalk with the microtubule-binding domain localized to the end Suter, 1996; Swan et al., 1999; Mach and Lehmann, 1997; Bullock (Gee et al., 1997; Burgess et al., 2003; Kon et al., 2011). AAA1 is and Ish-Horowicz, 2001). In mammals, there are two BICD the primary site of ATP hydrolysis, while the nucleotide state orthologs, BICD1 and BICD2, as well as two related proteins, of AAA3 has an allosteric effect on the motile properties of BICDR1 and BICDR2, which are slightly shorter. BICD proteins dynein (Takahide Kon et al., 2004; DeWitt et al., 2015; Nicholas form dimers characterized by long coiled-coil domains. Cryo-EM et al., 2015). analysis of the N-terminus of BICD2 shows an extended coiled-coil Dynein requires the co-factor dynactin, a 1 MDa, 23-subunit of ∼250 amino acid residues that extends for ∼30 nm and docks complex, the first identified activator for dynein and essential for onto the Arp1 filament of dynactin (Chowdhury et al., 2015; most cellular functions of the motor (see poster). The core of Urnavicius et al., 2015). BICD2 also interacts with the N-terminal dynactin is comprised of an ∼37-nm-long actin-like filament tail of the DHC (Chowdhury et al., 2015; Urnavicius et al., 2015) called the Arp1 filament (Schroer, 2004), which is composed of and the dynein LIC1, via coiled-coil interactions (Schroeder et al., eight Arp1 subunits (also known as ACTR1A), one β-actin 2014; Lee et al., 2018). Together, these interactions enhance the molecule and one Arp11 molecule (also known as ACTR10). affinity of the dynein–dynactin interaction (Splinter et al., 2012; Arp11 interacts with p25, p27 and p62 (also known as DCTN5, McKenney et al., 2014; Schlager et al., 2014a). While BICD2 is DCTN6 and DCTN4, respectively) to form the pointed-end mainly found in a complex with one dynein and one dynactin, complex of dynactin. The barbed end of the Arp1 filament within BICDR1 can recruit two dynein dimers to a single dynactin, which dynactin is capped by actin capping protein, a heterodimer of a further enhances the force and velocity of the motor complex CapZα and CapZβ family protein. A shoulder complex sits on the (Urnavicius et al., 2018; Grotjahn et al., 2018; Schlager et al., barbed-end, comprised of two copies of p24 (DCTN3), four 2014b). In mammalian cells, BICD proteins have been implicated copies of p50 (dynamtin or DCTN2) and two copies of p150Glued in Golgi vesicle transport via a C-terminal interaction with the (DCTN1) (Chowdhury et al., 2015; Urnavicius et al., 2015). The small GTPase Rab6 proteins (Hoogenraad et al., 2001; Matanis pointed-end of dynactin has been suggested to play a role in et al., 2002; Schlager et al., 2010; Short et al., 2002). BICD2 has facilitating cargo interaction (Zhang et al., 2011; Yeh et al., 2012; also been implicated in nuclear positioning (Splinter et al., 2012; Qiu et al., 2018) while the p150Glued subunit has an independent Hu et al., 2013). Furthermore, the C-terminal region of BICD and ATP-insensitive microtubule-binding domain (Waterman- proteins can bend back on itself to produce an autoinhibited state Storer et al., 1995). (Terawaki et al., 2015; Liu et al., 2013; Wharton and Struhl, 1989; Urnavicius et al., 2015), which can be relieved by cargo binding Activation of dynein (Liu et al., 2013; Huynh and Vale, 2017; McClintock et al., 2018; Dynein is responsible for the long-distance transport of many Sladewski et al., 2018), suggesting an efficient mechanism to cargos, some of which display highly processive motility. In vitro, regulate dynein motility. isolated or recombinant mammalian dynein is poorly processive Members of the Hook protein family activate dynein in a similar unless bound to a bead or other surface. Electron microscopy (EM) manner. There are three Hook proteins expressed in mammalian studies have indicated that in the absence of binding partners, cells, characterized by three conserved regions: a globular mammalian dynein is found in an auto-inhibited or phi state, which N-terminal Hook domain, a central coiled-coil domain that drives has a low affinity for microtubules (Torisawa et al., 2014; Zhang dimerization and forms a 31-nm helix that aligns along the Arp1 et al., 2017). In the presence of dynactin and a coiled-coil activating filament, and a divergent, predicted unstructured C-terminal domain adaptor, such as Bicaudal D (BICD) protein 2 (BICD2), the motor thought to mediate cargo binding (Walenta et al., 2001; Lee et al., Journal of Cell Science 2 CELL SCIENCE AT A GLANCE Journal of Cell Science (2019) 132, jcs227132.