[Show abstract][Hide abstract] ABSTRACT: The vacuolar (H+)-ATPase (V-ATPase) is crucial for maintenance of the acidic microenvironment in intracellular organelles, whereas its membrane-bound
V0-sector is involved in Ca2+-dependent membrane fusion. In the secretory pathway, the V-ATPase is regulated by its type I transmembrane and V0-associated accessory subunit Ac45. To execute its function, the intact-Ac45 protein is proteolytically processed to cleaved-Ac45
thereby releasing its N-terminal domain. Here, we searched for the functional domains within Ac45 by analyzing a set of deletion
mutants close to the in vivo situation, namely in transgenic Xenopus intermediate pituitary melanotrope cells. Intact-Ac45 was poorly processed and accumulated in the endoplasmic reticulum of
the transgenic melanotrope cells. In contrast, cleaved-Ac45 was efficiently transported through the secretory pathway, caused
an accumulation of the V-ATPase at the plasma membrane and reduced dopaminergic inhibition of Ca2+-dependent peptide secretion. Surprisingly, removal of the C-tail from intact-Ac45 caused cellular phenotypes also found for
cleaved-Ac45, whereas C-tail removal from cleaved-Ac45 still allowed its transport to the plasma membrane, but abolished V-ATPase
recruitment into the secretory pathway and left dopaminergic inhibition of the cells unaffected. We conclude that domains
located in the N- and C-terminal portions of the Ac45 protein direct its trafficking, V-ATPase recruitment and Ca2+-dependent-regulated exocytosis.
[Show abstract][Hide abstract] ABSTRACT: The vacuolar (H+)-ATPase (V-ATPase) is the main regulator of intraorganellar pH and in neuroendocrine cells is controlled by its accessory subunit, Ac45. Here, we report the discovery of the first isoform of a V-ATPase accessory subunit, namely an Ac45-like protein, denoted Ac45LP. Phylogenetic analysis revealed a lineage-dependent evolutionary history: Ac45 is absent in birds, and Ac45LP is absent in placental mammals, whereas all other tetrapod species contain both genes. In contrast to Ac45, Ac45LP is not proteolytically cleaved, a prerequisite for proper Ac45 routing. Intriguingly, Xenopus Ac45LP mRNA was expressed in developing neural tissue and in neural crest cells. In adult Xenopus, Ac45 mRNA is widely expressed mostly in neuroendocrine tissues, while Ac45LP mRNA expression was found to be restricted to the kidney and the lung. This novel Ac45LP may provide additional possibilities for V-ATPase regulation during neurodevelopment as well as in kidney and lung cells.
Electronic supplementary material
The online version of this article (doi:10.1007/s00018-009-0200-6) contains supplementary material, which is available to authorized users.
Cellular and Molecular Life Sciences CMLS 11/2009; 67(4):629-40. DOI:10.1007/s00018-009-0200-6 · 5.81 Impact Factor