Analysis in vivo using a new method, ARGO (Analysis of Red-Green Offset), reveals complexity and cell-type specificity in presynaptic turnover of synaptic vesicle protein Synaptogyrin/SNG-1.

In long-lived cells such as neurons, proteostasis involves the regulated degradation and replacement of proteins to ensure their quality and appropriate abundance. Synaptic vesicle (SV) protein turnover in neurons is important for controlling the SV pool size and sustaining appropriate levels of neurotransmission; yet, it is incompletely understood, partly due to limited tools for quantifying protein turnover in vivo. We present ARGO (Analysis of Red-Green Offset), a fully genetically encoded ratiometric fluorescence imaging method that visualizes and quantifies protein turnover with subcellular resolution in vivo. ARGO is inexpensive, modular, and scalable for use in genetically tractable experimental organisms. Using ARGO, we examine the turnover of Synaptogyrin/SNG-1, an evolutionarily conserved, integral SV protein, in Caenorhabditis elegans neurons. We show that the SNG-1 turnover rate is consistent across presynapses within a single neuron but varies between neuron classes. Notably, we find SNG-1 can exist in two distinct, nonintermixing populations within each presynapse. Furthermore, we present an initial mutant analysis of uba-1, the sole E1 ubiquitin ligase in C. elegans, demonstrating ARGO's capability to detect slowed SNG-1 turnover even though steady-state SNG-1 abundance is unchanged compared with wild-type. These results provide new hints for the regulation of the SV pool size.