Updated project metadata. The biological significance of circular RNAs remains largely unexplored due to the lack of loss-of-function animal models. In this study, we focused on circTulp4, a highly abundant circRNA that is enriched in the brain and synaptic compartments. We created a circTulp4-deficient (CD) mouse model and conducted a comprehensive phenotypic analysis. To test whether the electrophysiological phenotype observed in CD mice was explained by an alteration in the levels of synaptic proteins, we prepared pure synaptosomal fractions and conducted a quantitative proteomic analysis. Label-free quantification of CD and wildtype synaptic proteomes was performed by using an ion mobility-enhanced data-independent acquisition (DIA) workflow with alternating low and elevated energy (referred to as UDMSE). We quantified >1800 proteins and concluded that the protein composition of the synaptosomal compartment is preserved in CD mice.