Molecular mechanisms of cutis laxa– and distal renal tubular acidosis–causing mutations in V-ATPase a subunits, ATP6V0A2 and ATP6V0A4

Abstract

The <i>a</i> subunit is the largest of 15 different subunits that make up the vacuolar H⁺-ATPase (V-ATPase) complex, where it functions in proton translocation. In mammals, this subunit has four paralogous isoforms, <i>a</i>1-<i>a</i>4, which may encode signals for targeting assembled V-ATPases to specific intracellular locations. Despite the functional importance of the <i>a</i> subunit, its structure remains controversial. By studying molecular mechanisms of human disease-causing missense mutations within <i>a</i> subunit isoforms, we may identify domains critical for V-ATPase targeting, activity and/or regulation. cDNA-encoded FLAG-tagged human wildtype ATP6V0A2 (<i>a</i>2) and ATP6V0A4 (<i>a</i>4) subunits and their mutants, <i>a</i>2^P405L (causing cutis laxa), and <i>a</i>4^R449H and <i>a</i>4^G820R (causing renal tubular acidosis, dRTA), were transiently expressed in HEK 293 cells. <i>N</i>-Glycosylation was assessed using endoglycosidases, revealing that <i>a</i>2^P405L, <i>a</i>4^R449H, and <i>a</i>4^G820R were fully <i>N</i>-glycosylated. Cycloheximide (CHX) chase assays revealed that <i>a</i>2^P405L and <i>a</i>4^R449H were unstable relative to wildtype. <i>a</i>4^R449H was degraded predominantly in the proteasomal pathway, whereas <i>a</i>2^P405L was degraded in both proteasomal and lysosomal pathways. Immunofluorescence studies disclosed retention in the endoplasmic reticulum and defective cell-surface expression of <i>a</i>4^R449H and defective Golgi trafficking of <i>a</i>2^P405L Co-immunoprecipitation studies revealed an increase in association of <i>a</i>4^R449H with the <i>V</i>₀ assembly factor VMA21, and a reduced association with the <i>V</i>₁ sector subunit, ATP6V1B1 (B1). For <i>a</i>4^G820R, where stability, degradation, and trafficking were relatively unaffected, 3D molecular modeling suggested that the mutation causes dRTA by blocking the proton pathway. This study provides critical information that may assist rational drug design to manage dRTA and cutis laxa.