NuMA and Ninein: Dynein Cargo-Adaptors Without a Classical Cargo

Dynein is a minus-end-directed microtubule motor that transports a variety of cargoes. Cargo specificity is mediated by a class of adaptor proteins that bind to the interface between dynein and dynactin, along the length of the Arp1 filament of dynactin, and that co-activate the motor. NuMA, ninein, and ninein-like protein (Nlp) are cargo-adaptors that are involved in microtubule organization, rather than carrying portable cargoes. At the same time, ninein and Nlp are believed to be anchorage factors for gamma-tubulin ring complexes to the centrosome. Here, we discuss recent findings on the interaction of NuMA and ninein with the dynein/dynactin complex, and how these findings challenge earlier concepts on ninein-dependent microtubule organization via gamma-tubulin complexes. We do not intend to provide an encyclopedic review on NuMA and ninein, but rather develop a hypothesis about how conformational changes may regulate the activities and binding specificities of these two proteins.