The present disclosure relates to compositions and methods for the diagnosis and treatment or prevention of proteinopathies, particularly MUC1-associated kidney disease (ADTKD-MUC1 or MKD), Retinitis Pigmentosa (e.g., due to rhodopsin mutations), autosomal dominant tubulo-interstitial kidney disease due to UMOD mutation(s) (ADTKD-UMOD), and other forms of toxic proteinopathies resulting from mutant protein accumulation in the ER or other secretory pathway compartments and/or vesicles, among others. The disclosure also identifies and provides TMED9-binding agents as capable of treating or preventing proteinopathies of the secretory pathway, and further provides methods for identifying additional TMED9-binding agents.