scussion Here, we present the identification and characterization of a novel serine-threonine kinase termed Serine-Arginine Protein Kinase at 79D that PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/1986172 colocalizes with the T-barassociated protein Brp in both the axon and at the mature AZ. SRPK79D is one of very few proteins known to localize to T-bars or ribbon-like structures at the AZ and is the only known kinase to localize to this site . We further provide genetic evidence that SRPK79D functions to represses the premature assembly of T-bars in axons. In particular, we show that loss-offunction mutations in srpk79D cause the appearance of T-barlike protein aggregates throughout peripheral axons, and we are able to rule out the possibility that this is an indirect consequence of impaired axonal transport. The appearance of ectopic T-bars is highly specific since numerous other synaptic proteins and mitochondria are normally distributed in the neuron and are normally trafficked to the presynaptic nerve terminal in SRPK-Dependent Control of T-Bar Assembly the srpk79D mutant background. Thus, SRPK79D appears to have a specific function in Aglafoline repressing T-bar assembly prior to the AZ, consistent with 
the strong colocalization of SRPK79D protein with Brp and T-bar structures. Finally, we also uncover a potential function for SRPK79D at the AZ where it is observed to colocalize with Brp. SRPK79D lossof-function mutations do not alter the number, density, or organization of Brp puncta at the synapse and do not alter synaptic function. This is consistent with a negative regulatory role for SRPK79D during T-bar assembly and indicates that once SRPK79D-dependent repression of T-bar assembly is relieved, AZ assembly proceeds normally. Overexpression of SRPK79D, however, severely disrupts neurotransmission. The defect in presynaptic release is correlated with a disruption of Brp puncta organization and integrity. These phenotypes are consistent with a function for SRPK79D as a negative regulator of T-bar assembly and AZ maturation. SRPK79D is a member of the SRPK family of constitutively active cytoplasmic serine-threonine kinases that target serineargininerich domains of SR proteins. Thus, it is interesting to postulate what the relevant kinase target might be. Given that SRPK79D and Brp colocalize, an obvious candidate is the Brp protein itself. However, the Brp protein does not have a consensus SR domain, and decreasing the genetic dosage of srpk79D does not potentiate axonal Brp accumulations that appear upon Brp overexpression . As such, Brp may not be the direct target of SRPK79D kinase activity. We hypothesize, therefore, that SRPK79D colocalizes with Brp and another putative SR protein that is the direct target of SRPK79D kinase activity. Potential Models for SRPK79D-Dependent Negative Regulation of T-Bar Assembly The best-characterized role for SRPKs is in controlling the subcellular localization of SR proteins, thereby regulating their nuclear pre-mRNA splicing activity. More recently, SR protein involvement in several cytoplasmic mRNA regulatory roles has been reported. In particular, a phosphorylationdependent role for SR proteins has been reported in both Drosophila and mammalian cell culture. It is interesting to speculate that the function of SRPK79D to prevent premature T-bar assembly might be related to the established function of SRPKs and SR-domain-containing proteins during RNA binding, processing, and translation. One interesting possibility is that RNA species are resident at t
