Required for the assembly of the V0 complex of the vacuolar ATPase (V-ATPase) in the endoplasmic reticulum. Defects in VMA21 are the cause of X-linked myopathy with excessive autophagy (MEAX). MEAX is a childhood-onset disease characterized by progressive vacuolation and atrophy of skeletal muscle. It is inherited in recessive fashion, affecting boys and sparing carrier females. Onset is in childhood, and patients exhibit weakness of the proximal muscles of the lower extremities, progressing slowly to involve other skeletal muscle groups over time. Other organs including the heart and brain are clinically unaffected. Phenotype is due to an increase of lysosomal pH from 4.7 to 5.2, which reduces lysosomal degradative ability and blocks autophagy. This reduces cellular free amino acids, which up-regulates the mTOR pathway and mTOR-dependent macroautophagy, resulting in proliferation of large and ineffective autolysosomes that engulf sections of cytoplasm, merge together, and vacuolate the cell. Belongs to the VMA21 family. 2 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.
Protein type: Membrane protein, integral; Membrane protein, multi-pass