25.3 : Polarity of the Cytoskeleton

The intrinsic polarity of cells can be primarily attributed to two factors- i) the asymmetric accumulation of mobile components such are regulatory molecules and subcellular components across the cell and ii) the orientation of polar cytoskeletal filaments that make up the cytoskeletal networks, specifically microfilaments, and microtubules arranged along the axis of polarity. Interactions between the cytoskeletal filaments are crucial for the establishment and maintenance of the polar nature of the cell.

Other factors that contribute to cytoskeletal polarity include the structure of monomers, the structure of individual filaments, the filament’s rate of assembly or disassembly, accessory proteins associated with cytoskeletal filaments, and the overall asymmetric distribution of these filaments within the cell. Microtubules and microfilaments are inherently polar and contribute through all these factors, whereas intermediate filaments are non-polar and contribute only through their asymmetric distribution in a cell.

Cellular Functions Based on Cytoskeleton Polarity

The polarity of cytoskeletal filaments regulates cell motility and transport of molecules within the cell. A cell moves by repeatedly extending and forming an attachment at the front, followed by detachment and retraction of the rear end. The polarity of the microfilaments directs the directionality of such protrusions. In migrating cells, F-actins dominate at the migrating front and promote the formation of lamellipodia or filopodia—membrane protrusions essential for cell movement.

Vesicular transport by motor proteins is regulated by microtubule polarity. Microtubules determine the directionality of cargoes carried by kinesin and dynein. While kinesin moves towards the cell’s periphery with the plus-end of microtubules, dynein moves towards the nucleus with the minus end.