The microtubule-based motor protein cytoplasmic dynein is involved in transporting a diverse array of cargo, allowing cells to organize their contents, move, divide, and respond to stimuli. Dyneins are a family of "ATPases Associated with diverse cellular Activities" (AAA+) motors that move towards the minus end of microtubules.
Dynein holoenzymes comprise two ~500 kDa motor (or "head") proteins containing heavy chain subunits and at least six other polypeptides.
(Source: Wiki Commons)
Cytoplasmic dynein transports intracellular cargos in interphase cells and mediates spindle assembly and chromosome positioning during cell division. Other dyneins transport cargo molecules in cilia and power cilia beating. Historically, dyneins are the least studied cytoskeletal motor proteins due to the difficulties in studying active dynein complexes.
In 2012 Qui et al. reported a new model for studying the dynein stepping mechanism. The researchers combined two-color, single-molecule microscopy with high-precision, two-dimensional tracking for the study. The study found that dynein has a variable stepping pattern distinct from other cytoskeletal motor proteins. Processive cytoskeletal motors usually use "hand-over-hand" mechanisms. Whereas dynein has a stochastic or randomly determined hard-to-predict stepping pattern when its two motor domains are close together. According to the study, coordination emerges as the distance between motor domains increases. A tension-based mechanism governs these steps, allowing the tuning of dynein for all its cellular functions.
To enable the study, the researchers performed a two-dimensional analysis of dynein stepping by tracking GST-dynein homodimers labeled with a single Qdot 655 placed on either tail domain via an N-terminal HaloTag or a single motor domain containing a C-terminal HaloTag. The study found that dynein's step size was larger than the previously reported 1-D step size, and many steps had an off-axis component. Also, steps were equally likely to be to the left or the right. Dynein's stepping behavior showed significant variability and flexibility. In the off-axis component, some steps are backward. Dynein's two motor domains can step independently. Qui et al. proposed that this flexibility allows dynein to navigate a crowded cytoplasm and obstacles on microtubules, providing a molecular explanation for the observation that dynein can better navigate obstacles than kinesin motors. According to Qui et al., this apparent plasticity of the dynein stepping mechanism suggests that additional layers of regulation may be used to accomplish different cell biological functions. In eukaryotic cells, dynein transports dozens, if not hundreds, of varying cargo, but there is only a single gene encoding cytoplasmic dynein 1 in all sequenced eukaryotic genomes.
Bio-Synthesis Inc. prepared the Cy3B-Halotag.
Reference
Qiu W, Derr ND, Goodman BS, Villa E, Wu D, Shih W, Reck-Peterson SL. Dynein achieves processive motion using both stochastic and coordinated stepping. Nat Struct Mol Biol. 2012 Jan 8;19(2):193-200. [PMC]