Polarity of the Cytoskeleton

Microfilaments and microtubules are polar because their protein monomers are asymmetric and uniformly oriented during polymerization. G-actin monomers have two distinct domains that align in the same direction, creating plus and minus ends with different polymerization rates. Similarly, alpha-beta-tubulin dimers orient uniformly, with the minus end exposing free alpha-tubulins and the plus end exposing free beta-tubulins.

Cytoskeletal filament polarity directs motor proteins like kinesin and dynein along specific pathways. Kinesin moves toward the cell's periphery along the plus end of microtubules, while dynein moves toward the nucleus along the minus end. This directional movement allows motor proteins to transport cargos and vesicles to precise cellular locations.

Actin filaments have different polymerization rates at their plus and minus ends because of the asymmetric structure and uniform orientation of G-actin monomers. The structural differences between the two ends create distinct binding environments, causing monomers to add faster at the plus end than the minus end. This rate difference is fundamental to the filament's polarity.

Microfilament polarity directs the formation of membrane protrusions essential for cell migration. F-actins accumulate at the migrating front and promote the formation of lamellipodia or filopodia, which extend forward. The cell then attaches at the front and retracts the rear, with microfilament polarity determining the direction of these protrusions and overall cell movement.

Intermediate filaments are non-polar structures, unlike microfilaments and microtubules, which are inherently polar. While polar filaments contribute to cell polarity through monomer structure, filament structure, assembly rates, and accessory proteins, intermediate filaments contribute only through their asymmetric distribution within the cell.

Cell polarity is established and maintained through asymmetric accumulation of regulatory molecules and subcellular components across the cell, combined with the orientation of polar cytoskeletal filaments. Accessory proteins associated with cytoskeletal filaments and the overall asymmetric distribution of these filaments within the cell also contribute significantly to maintaining the cell's polar nature.

Microtubule polarity determines the directionality of cargo transport by establishing distinct plus and minus ends. Kinesin motors recognize the plus end and transport vesicles toward the cell periphery, while dynein recognizes the minus end and transports cargo toward the nucleus. This polarity-based system ensures cargos reach their correct cellular destinations.