btn1 affects cytokinesis and cell-wall deposition by independent mechanisms, one of which is linked to dysregulation of vacuole pH

btn1, the Schizosaccharomyces pombe orthologue of the human Batten-disease gene CLN3, is involved in vacuole pH homeostasis. We show that loss of btn1 also results in a defective cell wall marked by sensitivity to zymolyase, a -glucanase. The defect can be rescued by expression of Btn1p or CLN3, and the extent of the defect correlates with disease severity. The vacuole and cell-wall defects are linked by a common pH-dependent mechanism, because they are suppressed by growth in acidic pH and a similar glucan defect is also apparent in the V-type H⁺ ATPase (v-ATPase) mutants vma1 and vma3. Significantly, Btn1p acts as a multicopy suppressor of the cell-wall and other vacuole-related defects of these v-ATPase-null cells. In addition, Btn1p is required in a second, pH-independent, process that affects sites of polarised growth and of cell-wall deposition, particularly at the septum, causing cytokinesis problems under normal growth conditions and eventual cell lysis at 37°C. Thus, Btn1p impacts two independent processes, which suggests that Batten disease is more than a pH-related lysosome disorder.