Molecular model of the hetero 3-mer kinesin-like apo protein KIF1A (green) complexed with tubulin alpha-1B chain (pink) and tubulin beta-2B chain (blue) from sus scrofa. Kinesin activity is regulated by nucleotide binding. In the "APO" state, the nucleotide binding pocket is empty. Microtubules are hollow, cylindrical structure made of tubulin proteins that form part of the cytoskeleton. Kinesin motor proteins walk along microtubules to transport various cellular cargo.