As the primary microtubule-organizing centers, centrosomes require γ-tubulin for microtubule nucleation and organization. Located in close vicinity to centrosomes, the Golgi complex is another microtubule-organizing organelle in interphase cells. CDK5RAP2 is a γ-tubulin complex-binding protein on centrosomes and functions in γ-tubulin attachment to centrosomes. In this study, I find CDK5RAP2 localizes to the Golgi in a centrosome dependent manner. The targeting of CDK5RAP2 is mediated by its C-terminus that binds to pericentrin and AKAP450. The pericentrin and AKAP450 bindings are respectively required for the centrosome/Golgi and the Golgi localization of CDK5RAP2. The C-terminus of CDK5RAP2 has a calmodulin binding motif that is necessary for pericentrin and AKAP450 bindings. The bind...[ Read more ]

As the primary microtubule-organizing centers, centrosomes require γ-tubulin for microtubule nucleation and organization. Located in close vicinity to centrosomes, the Golgi complex is another microtubule-organizing organelle in interphase cells. CDK5RAP2 is a γ-tubulin complex-binding protein on centrosomes and functions in γ-tubulin attachment to centrosomes. In this study, I find CDK5RAP2 localizes to the Golgi in a centrosome dependent manner. The targeting of CDK5RAP2 is mediated by its C-terminus that binds to pericentrin and AKAP450. The pericentrin and AKAP450 bindings are respectively required for the centrosome/Golgi and the Golgi localization of CDK5RAP2. The C-terminus of CDK5RAP2 has a calmodulin binding motif that is necessary for pericentrin and AKAP450 bindings. The binding motif, but not the calmodulin binding is indispensible for the targeting of CDK5RAP2 to the centrosome and Golgi. Myomegalin was initially discovered in rat muscle as a phosphodiesterase-4D binding protein with unknown function and unknown expression profile in other tissues or cells. In muscles, myomegalin is 230-250kDa protein at the Z-disk. In an attempt to detect the expression of MMG in HeLa cell, I discovered a novel myomegalin isoform (MMG8) that is expressed in all cell lines examined and in all tissues other than muscles. MMG8 localizes to the Golgi where it binds to AKAP450 for mutual stabilization and is as such involved microtubule nucleation from the Golgi. MMG8 on the Golgi also binds to EB1 through which it captures microtubules. MMG8 weakly binds to syntaxin-5, a most important SNARE protein during ER-to-Golgi trafficking. Inhibition of syntaxin-5 SNARE cycle results in the delocalization of MMG8 from the Golgi. Suppression of MMG8 expression resultes in cytosol accumulation of COPI vesicle proteins and delay of Golgi recovery after BFA washout. With all these observations, I propose a function of MMG8 in coordinating the microtubule-organization and syntaxin 5 SNAREs function on the Golgi.