Neuronal endolysosomal acidification relies on interactions between transmembrane protein 184B (TMEM184B) and the vesicular proton pump.

Disruption of endolysosomal acidification is a hallmark of several neurodevelopmental and neurodegenerative disorders. Impaired acidification causes accumulation of toxic protein aggregates and disrupts neuronal homeostasis, yet the molecular mechanisms regulating endolysosomal pH in neurons remain poorly understood. A critical regulator of lumenal acidification is the vacuolar ATPase (V-ATPase), a proton pump whose activity depends on dynamic assembly of its V0 and V1 subdomains. In this study, we identify transmembrane protein 184B (TMEM184B) as a novel regulator of endolysosomal acidification in neurons. TMEM184B is an evolutionarily conserved 7-pass transmembrane protein required for synaptic structure and function, and sequence variation in TMEM184B causes neurodevelopmental disorders, but the mechanism for this effect is unknown. We performed proteomic analysis of TMEM184B-interacting proteins and identified enrichment of components involved in endosomal trafficking and function, including the V-ATPase. TMEM184B localizes to early and late endosomes, further supporting a role in the endosomal system. Loss of TMEM184B results in significant reductions in endolysosomal acidification within cultured mouse cortical neurons. This alteration in pH is associated with impaired assembly of the V-ATPase V0 and V1 subcomplexes in the TMEM184B mutant mouse brain, suggesting a mechanism by which TMEM184B promotes flux through the endosomal pathway. Overall, these findings identify a new contributor in maintaining endosomal function and provide a mechanistic basis for disrupted neuronal function in human TMEM184B-associated nervous system disorders. Significance Statement: Endolysosomal acidification is essential for neuronal protein homeostasis, yet its regulation in neurons remains poorly understood. Here, we identify TMEM184B as a key regulator of this process, establishing its first known cellular role. We show that TMEM184B interacts with vacuolar ATPase (V-ATPase) components and promotes the assembly of its V0 and V1 subdomains, facilitating lumenal acidification. Loss of TMEM184B disrupts endolysosomal pH in neurons, potentially impairing proteostasis. These findings reveal a critical function for TMEM184B in neuronal maintenance and provide mechanistic insight into its link to neurological disorders. This work advances our understanding of endolysosomal regulation and suggests TMEM184B regulation could improve outcomes in diseases involving lysosomal dysfunction.