Functional dissection of the dynein motor domain

Genetic analysis of the cytoplasmic dynein subunit families

Cytoplasmic dyneins, the principal microtubule minus-end-directed motor proteins of the cell, are involved in many essential cellular processes. The major form of this enzyme is a complex of at least six protein subunits, and in mammals all but one of the subunits are encoded by at least two genes. Here we review current knowledge concerning the subunits, their interactions, and their functional roles as derived from biochemical and genetic analyses. We also carried out extensive database searches to look for new genes and to clarify anomalies in the databases. Our analysis documents evolutionary relationships among the dynein subunits of mammals and other model organisms, and sheds new light on the role of this diverse group of proteins, highlighting the existence of two cytoplasmic dynein complexes with distinct cellular roles.

The structure of microtubule motor proteins

Microtubules are the intracellular tracks for two classes of motor proteins: kinesins and dyneins. During the past few years, the motor domain structures of several kinesins from different organisms have been determined by X-ray crystallography. Compared with kinesins, dyneins are much larger proteins and attempts to crystallize them have failed so far. Structural information about these proteins comes mostly from electron microscopy. In this chapter, we mainly focus on the crystal structures of kinesin motor domains.

Design and regulation of the AAA+ microtubule motor dynein

Dyneins are highly complex molecular motors that transport their attached cargo towards the minus end of microtubules. These enzymes are required for many essential motile activities within the cytoplasm and also power eukaryotic cilia and flagella. Each dynein contains one or more heavy chain motor units that consist of an N-terminal stem domain that is involved in cargo attachment, and six AAA+ domains (AAA1-6) plus a C-terminal globular segment that are arranged in a heptameric ring. At least one AAA+ domain (AAA1) is capable of ATP binding and hydrolysis, and the available data suggest that one or more additional domains also may bind nucleotide. The ATP-sensitive microtubule binding site is located at the tip of a 10nm coiled coil stalk that emanates from between AAA4 and AAA5. The function of this motor both in the cytoplasm and the flagellum must be tightly regulated in order to result in useful work. Consequently, dyneins also contain a series of additional components that serve to define the cargo-binding properties of the enzyme and which act as sensors to transmit regulatory inputs to the motor units. Here we describe the two basic dynein designs and detail the various regulatory systems that impinge on this motor within the eukaryotic flagellum.

The third P-loop domain in cytoplasmic dynein heavy chain is essential for dynein motor function and ATP-sensitive microtubule binding

Sequence comparisons and structural analyses show that the dynein heavy chain motor subunit is related to the AAA family of chaperone-like ATPases. The core structure of the dynein motor unit derives from the assembly of six AAA domains into a hexameric ring. In dynein, the first four AAA domains contain consensus nucleotide triphosphate-binding motifs, or P-loops. The recent structural models of dynein heavy chain have fostered the hypothesis that the energy derived from hydrolysis at P-loop 1 acts through adjacent P-loop domains to effect changes in the attachment state of the microtubule-binding domain. However, to date, the functional significance of the P-loop domains adjacent to the ATP hydrolytic site has not been demonstrated. Our results provide a mutational analysis of P-loop function within the first and third AAA domains of the Drosophila cytoplasmic dynein heavy chain. Here we report the first evidence that P-loop-3 function is essential for dynein function. Significantly, our results further show that P-loop-3 function is required for the ATP-induced release of the dynein complex from microtubules. Mutation of P-loop-3 blocks ATP-mediated release of dynein from microtubules, but does not appear to block ATP binding and hydrolysis at P-loop 1. Combined with the recent recognition that dynein belongs to the family of AAA ATPases, the observations support current models in which the multiple AAA domains of the dynein heavy chain interact to support the translocation of the dynein motor down the microtubule lattice.

Role of fungal dynein in hyphal growth, microtubule organization, spindle pole body motility and nuclear migration

Cytoplasmic dynein is a microtubule-associated motor protein with several putative subcellular functions. Sequencing of the gene (DHC1) for cytoplasmic dynein heavy chain of the filamentous ascomycete, Nectria haematococca, revealed a 4,349-codon open reading frame (interrupted by two introns) with four highly conserved P-loop motifs, typical of cytoplasmic dynein heavy chains. The predicted amino acid sequence is 78.0% identical to the cytoplasmic dynein heavy chain of Neurospora crassa, 70.2% identical to that of Aspergillus nidulans and 24.8% identical to that of Saccharomyces cerevisiae. The genomic copy of DHC1 in N. haematococca wild-type strain T213 was disrupted by inserting a selectable marker into the central motor domain. Mutants grew at 33% of the wild-type rate, forming dense compact colonies composed of spiral and highly branched hyphae. Major cytological phenotypes included (1) absence of aster-like arrays of cytoplasmic microtubules focused at the spindle pole bodies of post-mitotic and interphase nuclei, (2) limited post-mitotic nuclear migration, (3) lack of spindle pole body motility at interphase, (4) failure of spindle pole bodies to anchor interphase nuclei, (5) nonuniform distribution of interphase nuclei and (6) small or ephemeral Spitzenkorper at the apices of hyphal tip cells. Microtubule distribution in the apical region of tip cells of the mutant was essentially normal. The nonuniform distribution of nuclei in hyphae resulted primarily from a lack of both post-mitotic nuclear migration and anchoring of interphase nuclei by the spindle pole bodies. The results support the hypothesis that DHC1 is required for the motility and functions of spindle pole bodies, normal secretory vesicle transport to the hyphal apex and normal hyphal tip cell morphogenesis.