<Li> Kinesin has a similar structure to dynein . Kinesin is involved in the transport of a variety of intracellular cargoes, including vesicles, organelles, protein complexes, and mRNAs toward the microtubule's (+) end . </Li> <P> Some viruses (including retroviruses, herpesviruses, parvoviruses, and adenoviruses) that require access to the nucleus to replicate their genomes attach to motor proteins . </P> <P> Microtubule plus ends are often localized to particular structures . In polarized interphase cells, microtubules are disproportionately oriented from the MTOC toward the site of polarity, such as the leading edge of migrating fibroblasts . This configuration is thought to help deliver microtubule - bound vesicles from the Golgi to the site of polarity . </P> <P> Dynamic instability of microtubules is also required for the migration of most mammalian cells that crawl . Dynamic microtubules regulate the levels of key G - proteins such as RhoA and Rac1, which regulate cell contractility and cell spreading . Dynamic microtubules are also required to trigger focal adhesion disassembly, which is necessary for migration . It has been found that microtubules act as "struts" that counteract the contractile forces that are needed for trailing edge retraction during cell movement . When microtubules in the trailing edge of cell are dynamic, they are able to remodel to allow retraction . When dynamics are suppressed, microtubules cannot remodel and, therefore, oppose the contractile forces . The morphology of cells with suppressed microtubule dynamics indicate that cells can extend the front edge (polarized in the direction of movement), but have difficulty retracting their trailing edge . On the other hand, high drug concentrations, or microtubule mutations that depolymerize the microtubules, can restore cell migration but there is a loss of directionality . It can be concluded that microtubules act both to restrain cell movement and to establish directionality </P>

What structure consists of a 9+2 arrangement of microtubules and can be found on sperm cells