1. ¹Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas, USA
2. ²Department of Physiology, University of Texas Southwestern Medical Center, Dallas, Texas, USA
3. ³Eugene McDermott Center for Human Growth and Development, University of Texas Southwestern Medical Center, Dallas, Texas, USA
4. ⁴Charles and Jane Pak Center of Mineral Metabolism, University of Texas Southwestern Medical Center, Dallas, Texas, USA

Correspondence: OW Moe, Department of Internal Medicine, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, Texas 75390-8885, USA. E-mail: orson.moe@utsouthwestern.edu

Received 5 October 2007; Revised 8 January 2008; Accepted 8 January 2008; Published online 26 March 2008.

Abstract

Mutations in the B1 subunit of the multisubunit vacuolar ATPase cause autosomal-recessive distal renal tubular acidosis and sensorineural deafness. Here, we report a novel frameshift mutation that truncates the C-terminus of the human B1 subunit. This mutant protein failed to assemble with other subunits in the cytosol to form the complex that can be targeted to vesicular structures in mammalian cells. Loss of proton pump activity was demonstrated in a functional complementation assay in B-subunit null yeast. The mutation caused loss of a discreet C-terminal region critical for subunit interaction not related to the C-terminal PDZ motif. Co-expression studies failed to demonstrate dominant negative effects of this truncated mutant over wild-type B1. Analysis of 12 reported B1 subunit missense mutations showed one polymorphic allele had intact pump function, two point mutants had intact assembly but defective proton pumping, and the remaining nine had disrupted assembly with no pump function. One presumed polymorphic allele was actually an inactivating mutation. Our study shows that multiple mechanisms of pump dysfunction result from B1 subunit mutations with a common outcome being defective assembly. Polymorphisms of the B1 subunit in the general population may affect renal acidification and urinary chemistry.