We show here that an active Cdk5-p35 kinase is present in Golgi membranes, where it associates with a detergentinsoluble fraction containing actin. In addition, Cdk5-p35-dependent phosphorylation of α-PAK immunoreactive protein species was detected in Golgi membranes, as well as an interaction with the small GTPase, Cdc42. Moreover, antisense oligonucleotide suppression of Cdk5 or p35 in young cultured neurons, as well as inhibition of Cdk5 activity with olomoucine, blocks the formation of membrane vesicles from the Golgi apparatus. Taken together, these results show a novel subcellular localization of this kinase and suggest a role for Cdk5-p35 in membrane traffic during neuronal process outgrowth.
We used proximity‐dependent biotin identification (BioID) to find proteins that potentially interact with the major glial glutamate transporter, GLT‐1, and we studied how these interactions might affect its activity. GTPase Rac1 was one protein identified, and interfering with its GTP/GDP cycle in mixed primary rat brain cultures affected both the clustering of GLT‐1 at the astrocytic processes and the transport kinetics, increasing its uptake activity at low micromolar glutamate concentrations in a manner that was dependent on the effector kinase PAK1 and the actin cytoskeleton. Interestingly, the same manipulations had a different effect on another glial glutamate transporter, GLAST, inhibiting its activity. Importantly, glutamate acts through metabotropic receptors to stimulate the activity of Rac1 in astrocytes, supporting the existence of cross‐talk between extracellular glutamate and the astrocytic form of the GLT‐1 regulated by Rac1. CDC42EP4/BORG4 (a CDC42 effector) was also identified in the BioID screen, and it is a protein that regulates the assembly of septins and actin fibers, influencing the organization of the cytoskeleton. We found that GLT‐1 interacts with septins, which reduces its lateral mobility at the cell surface. Finally, the G‐protein subunit GNB4 dampens the activity of GLT‐1, as revealed by its response to the activator peptide mSIRK, both in heterologous systems and in primary brain cultures. This effect occurs rapidly and thus, it is unlikely to depend on cytoskeletal dynamics. These novel interactions shed new light on the events controlling GLT‐1 activity, thereby helping us to better understand how glutamate homeostasis is maintained in the brain.
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