The complex of outer-arm dynein light chain-1 and the microtubule-binding domain of the γ heavy chain shows how axonemal dynein tunes ciliary beating.

Axonemal dynein is a microtubule-based molecular motor that drives ciliary/flagellar beating in eukaryotes. In axonemal dynein, the outer-arm dynein (OAD) complex, which comprises three heavy chains (α, ß, and γ), produces the main driving force for ciliary/flagellar motility. It has recently been shown that axonemal dynein light chain-1 (LC1) binds to the microtubule-binding domain (MTBD) of OADγ, leading to a decrease in its microtubule-binding affinity. However, it remains unclear how LC1 interacts with the MTBD and controls the microtubule-binding affinity of OADγ. Here, we have used X-ray crystallography and pulldown assays to examine the interaction between LC1 and the MTBD, identifying two important sites of interaction in the MTBD. Solving the LC1-MTBD complex from Chlamydomonas reinhardtii at 1.7 Å resolution, we observed that one site is located in the H5 helix and that the other is located in the flap region that is unique to some axonemal dynein MTBDs. Mutational analysis of key residues in these sites indicated that the H5 helix is the main LC1-binding site. We modeled the ternary structure of the LC1-MTBD complex bound to microtubules based on the known dynein-microtubule complex. This enabled us to propose a structural basis for both formations of the ternary LC1-MTBD-microtubule complex and LC1-mediated tuning of MTBD binding to the microtubule, suggesting a molecular model for how axonemal dynein senses the curvature of the axoneme and tunes ciliary/flagellar beating.

Comparative Studies of Actin- and Rho-Specific ADP-Ribosylating Toxins: Insight from Structural Biology.

Mono-ADP-ribosylation is a major post-translational modification performed by bacterial toxins, which transfer an ADP-ribose moiety to a substrate acceptor residue. Actin- and Rho-specific ADP-ribosylating toxins (ARTs) are typical ARTs known to have very similar tertiary structures but totally different targets. Actin-specific ARTs are the A components of binary toxins, ADP-ribosylate actin at Arg177, leading to the depolymerization of the actin cytoskeleton. On the other hand, C3-like exoenzymes are Rho-specific ARTs, ADP-ribosylate Rho GTPases at Asn41, exerting an indirect effect on the actin cytoskeleton. This review focuses on the differences and similarities of actin- and Rho-specific ARTs, especially with respect to their substrate recognition and cell entry mechanisms, based on structural studies.

Rho GTPase Recognition by C3 Exoenzyme Based on C3-RhoA Complex Structure.

C3 exoenzyme is a mono-ADP-ribosyltransferase (ART) that catalyzes transfer of an ADP-ribose moiety from NAD(+) to Rho GTPases. C3 has long been used to study the diverse regulatory functions of Rho GTPases. How C3 recognizes its substrate and how ADP-ribosylation proceeds are still poorly understood. Crystal structures of C3-RhoA complex reveal that C3 recognizes RhoA via the switch I, switch II, and interswitch regions. In C3-RhoA(GTP) and C3-RhoA(GDP), switch I and II adopt the GDP and GTP conformations, respectively, which explains why C3 can ADP-ribosylate both nucleotide forms. Based on structural information, we successfully changed Cdc42 to an active substrate with combined mutations in the C3-Rho GTPase interface. Moreover, the structure reflects the close relationship among Gln-183 in the QXE motif (C3), a modified Asn-41 residue (RhoA) and NC1 of NAD(H), which suggests that C3 is the prototype ART. These structures show directly for the first time that the ARTT loop is the key to target protein recognition, and they also serve to bridge the gaps among independent studies of Rho GTPases and C3.

Regulation of motor activity of ciliary outer-arm dynein by the light chain 1; Implications from the structure of the light chain bound to the microtubule-binding domain of the heavy chain.

Ciliary bending movements are powered by motor protein axonemal dyneins. They are largely classified into two groups, inner-arm dynein and outer-arm dynein. Outer-arm dynein, which is important for the elevation of ciliary beat frequency, has three heavy chains (α, ß, and γ), two intermediate chains, and more than 10 light chains in green algae, Chlamydomonas. Most of intermediate chains and light chains bind to the tail regions of heavy chains. In contrast, the light chain LC1 was found to bind to the ATP-dependent microtubule-binding domain of outer-arm dynein γ-heavy chain. Interestingly, LC1 was also found to interact with microtubules directly, but it reduces the affinity of the microtubule-binding domain of γ-heavy chain for microtubules, suggesting the possibility that LC1 may control ciliary movement by regulating the affinity of outer-arm dyneins for microtubules. This hypothesis is supported by the LC1 mutant studies in Chlamydomonas and Planaria showing that ciliary movements in LC1 mutants were disordered with low coordination of beating and low beat frequency. To understand the molecular mechanism of the regulation of outer-arm dynein motor activity by LC1, X-ray crystallography and cryo-electron microscopy have been used to determine the structure of the light chain bound to the microtubule-binding domain of γ-heavy chain. In this review article, we show the recent progress of structural studies of LC1, and suggest the regulatory role of LC1 in the motor activity of outer-arm dyneins. This review article is an extended version of the Japanese article, The Complex of Outer-arm Dynein Light Chain-1 and the Microtubule-binding Domain of the Heavy Chain Shows How Axonemal Dynein Tunes Ciliary Beating, published in SEIBUTSU BUTSURI Vol. 61, p. 20-22 (2021).

Crystal structure of the stalk region of axonemal inner-arm dynein-d reveals unique features in the coiled-coil and microtubule-binding domain.

Axonemal dynein is an ATP-dependent microtubular motor protein responsible for cilia and flagella beating, and its dysfunction can cause diseases such as primary ciliary dyskinesia and sperm dysmotility. Despite its biological importance, structure-based mechanisms underlying axonemal dynein motors remain unclear. Here, we determined the X-ray crystal structure of the human inner-arm dynein-d (DNAH1) stalk region, which contains a long antiparallel coiled-coil and a microtubule-binding domain (MTBD), at 2.7 Å resolution. Notably, differences in the relative orientation of the coiled-coil and MTBD in comparison with other dyneins, as well as the diverse orientations of the MTBD flap region among various isoforms, lead us to propose a 'spike shoe model' with an altered stepping angle for the interaction between IAD-d and microtubules. Based on these findings, we discuss isoform-specific functions of the axonemal dynein stalk MTBDs.

Structural atlas of dynein motors at atomic resolution.

Dynein motors are biologically important bio-nanomachines, and many atomic resolution structures of cytoplasmic dynein components from different organisms have been analyzed by X-ray crystallography, cryo-EM, and NMR spectroscopy. This review provides a historical perspective of structural studies of cytoplasmic and axonemal dynein including accessory proteins. We describe representative structural studies of every component of dynein and summarize them as a structural atlas that classifies the cytoplasmic and axonemal dyneins. Based on our review of all dynein structures in the Protein Data Bank, we raise two important points for understanding the two types of dynein motor and discuss the potential prospects of future structural studies.