Establishing, maintaining and dynamically modifying synaptic connections is a key principle of neuronal computations. The underlying molecular resource of these processes is synaptic proteins. Tight control of protein levels across neurons is crucial to ensure synaptic function. Changes in the synaptic proteome have been associated with compromised synaptic function and in severe cases various diseases. Among the major factors in regulating protein levels are thought to be protein synthesis, trafficking and degradation. While there is substantial literature on protein synthesis, both trafficking and degradation are much less understood. Moreover, it is unknown how these processes are coordinated spatially and temporally within single neurons and across different cell types. To elucidate this entanglement, we analyzed data on the brain-wide turnover of the protein PSD-95 in the mouse brain [1] and combined them with a mathematical model. PSD-95 is a postsynaptic structural protein closely linked to synaptic strength. Our approach allows us to explain fundamental properties of the PSD-95 distribution in neurons and sheds light on how it is shaped by the individual contributions of protein synthesis, trafficking and degradation. Thereby we provide a new perspective on how intracellular dynamics and turnover shape the molecular basis of learning and memory.[1] Mohar B. et al., Brain-wide measurement of protein turnover with high spatial and temporal resolution. biorxiv (2022), doi.org/10.1101/2022.11.12.516226