Microtubule-independent and Protein Kinase A-mediated Function of Kinesin KIF17b Controls the Intracellular Transport of Activator of CREM in Testis (ACT)

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Kinesins are motor proteins that transport their cargos along microtubules in an ATP-dependent manner. The testis-specific kinesin KIF17b was shown to directly regulate cAMP-response element modulator (CREM)-dependent transcription by determining the subcellular localization of the activator of CREM in testis (ACT), the testis-specific coactivator of CREM in postmeiotic male germ cells. CREM is a crucial transcriptional regulator of many important genes required for spermatid maturation, as demonstrated by the complete block of sperm development at the first steps of spermiogenesis in crem-null mice. To better understand the complex regulation of postmeiotic germ cell differentiation, we further characterized the ACT-KIF17b interaction, the function of KIF17b, and the signaling pathways governing its action. In this study, we demonstrated that the abilities of KIF17b to shuttle between the nuclear and the cytoplasmic compartments and to transport ACT are neither dependent on its motor domain nor on microtubules, thus revealing a novel microtubule-independent function for kinesins. We also showed that the cyclic AMP-dependent protein kinase A mediates the phosphorylation of KIF17b, and this modification is important for its subcellular localization. These results indicate that cyclic AMP signaling controls CREM-mediated transcription in male germ cells through modification of KIF17b function.