The In Situ Structure and Distribution of V-ATPase in C. elegans Apical Membrane Stacks

The cuticle of Caenorhabditis elegans is an apical extracellular matrix composed primarily of collagens which protects the organism from external stresses and facilitates mobility. The cuticle directly contacts several epithelial cell structures including regular, repeating folds of the apical plasma membrane, known as apical membrane stacks (AMSs). Here, we examine the structure of AMSs using in situ cryo-electron tomography (cryo-ET). We find that V-ATPases are distributed across the cytoplasmic-facing membranes of the AMSs. We then determine the in situ structure of V-ATPase and show that it is highly conserved relative to other eukaryotic V-ATPases. Analysis of the distribution of V-ATPases on the AMSs shows that they form clusters in which individual V-ATPases are tightly packed but do not oligomerise. Our data also show that complete, fully assembled V-ATPases line the AMSs. Previous studies have established that V-ATPases can only act as ATP-dependent proton pumps in their fully assembled state. Our findings indicate that V-ATPases arranged along the AMSs pump protons directly into the extracellular matrix, suggesting that a primary function of AMSs is to acidify the cuticle.