Drosophila Syd-1, Liprin-α, and Protein Phosphatase 2A B′ Subunit Wrd Function in a Linear Pathway to Prevent Ectopic Accumulation of Synaptic Materials in Distal Axons

During synaptic development, presynaptic differentiation occurs as an intrinsic property of axons to form specialized areas of plasma membrane [active zones (AZs)] that regulate exocytosis and endocytosis of synaptic vesicles. Genetic and biochemical studies in vertebrate and invertebrate model systems have identified a number of proteins involved in AZ assembly. However, elucidating the molecular events of AZ assembly in a spatiotemporal manner remains a challenge. Syd-1 (synapse defective-1) and Liprin-alpha have been identified as two master organizers of AZ assembly. Genetic and imaging analyses in invertebrates show that Syd-1 works upstream of Liprin-alpha in synaptic assembly through undefined mechanisms. To understand molecular pathways downstream of Liprin-alpha, we performed a proteomic screen of Liprin-alpha -interacting proteins in Drosophila brains. We identify Drosophila protein phosphatase 2A (PP2A) regulatory subunit B' [Wrd (Well Rounded)] as a Liprin-alpha -interacting protein, and we demonstrate that it mediates the interaction of Liprin-alpha with PP2A holoenzyme and the Liprin-alpha -dependent synaptic localization of PP2A. Interestingly, loss of function in syd-1, liprin-alpha, or wrd shares a common defect in which a portion of synaptic vesicles, dense-core vesicles, and presynaptic cytomatrix proteins ectopically accumulate at the distal, but not proximal, region of motoneuron axons. Strong genetic data show that a linear syd-1/liprin-alpha/wrd pathway in the motoneuron antagonizes glycogen synthase kinase-3 alpha kinase activity to prevent the ectopic accumulation of synaptic materials. Furthermore, we provide data suggesting that the syd-1/liprin-alpha/wrd pathway stabilizes AZ specification at the nerve terminal and that such a novel function is independent of the roles of syd-1/liprin-alpha in regulating the morphology of the T-bar structural protein BRP (Bruchpilot).