Functional significance of the beta-subunit for heterodimeric P-type ATPases.

Author:

Chow D C1,Forte J G1

Affiliation:

1. Department of Molecular and Cell Biology, University of California, Berkeley 94720.

Abstract

We have reviewed the structural and functional role of the beta-subunit in a subfamily of the P-ATPases known as the alpha/beta-heterodimeric, cation-exchange ATPases. The subfamily consists of the various isoforms of Na+/K(+)-ATPase and H+/K(+)-ATPase, both of which pump a cation out of the cell (Na+ or H+, respectively) in recycle exchange for K+. Much of the earlier work has emphasized the functional activities of the alpha-subunit, which shares many characteristics with the broader P-ATPase family. It is now clear that the glycosylated beta-subunit is an essential component of the cation-exchange ATPase subfamily. All beta-subunit isoforms have three highly conserved disulfide bonds within the extracellular domain that serve to stabilize the alpha-subunit, alpha/beta interaction and functional activity of the holoenzyme. Evidence strongly suggests that the beta-subunit is involved in the K(+)-dependent reactions of the enzymes, such as the E1-E2 transition and K+ occlusion, and that the extracellular domain of the beta-subunit plays an important role in determining the kinetics of K+ interaction. In most vertebrate cells, the unassociated alpha-subunit is restricted to the endoplasmic reticulum (ER), and assembly of the alpha/beta complex occurs within the ER. Signals for exiting the ER and directing the correct intracellular trafficking are primarily determined by the beta-subunit; Na+/K(+)-ATPase typically terminates in the plasma membrane facing the basolateral membrane, whereas all isoforms of H+/K(+)-ATPase terminate in the apical membrane. The C-terminal extracellular domain of the beta-subunit is important for proper interaction with the alpha-subunit and for correct intracellular trafficking. Oligosaccharides on the beta-subunit are not essential for enzyme function, but do serve to enhance the efficiency of alpha/beta association by increasing the lifetime of the unassociated beta-subunit and the stability of the alpha/beta complex to tryptic attack. We propose that highly specialized glycosylation on the beta-subunit of the gastric H+/K(+)-ATPase may help to protect that enzyme from the harsh extracellular environment of the stomach.

Publisher

The Company of Biologists

Subject

Insect Science,Molecular Biology,Animal Science and Zoology,Aquatic Science,Physiology,Ecology, Evolution, Behavior and Systematics

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