Microtubules mediate the spatial organization of diverse membrane-trafficking systems. The HOOK proteins, HOOK1, HOOK2 and HOOK3, comprise a family of cytosolic coiled-coil proteins that contain conserved N-terminal domains, which attach to microtubules; and more divergent C-terminal domains, which mediate binding to organelles. HOOK1, a cytoskeletal linker protein, may play a role in endocytic membrane trafficking. It exists as a homodimer, most likely mediated through its central coiled-coil domain. HOOK1 interacts with VPS18 and is required for spermatid differentiation, in which it is most likely involved in the positioning of the manchette microtubules and the flagellum. HOOK1 localizes primarily to the cytoplasm and does not associate with the Golgi complex, unlike HOOK3, which participates in the organization of the cis-Golgi compartment.