Quaternary structure of (Na+ + K+)-dependent adenosine triphosphatase.

(Na+ + K+)-dependent adenosine triphosphatase (NaK-ATPase) consists of two polypeptide chains, a large polypeptide with a molecular weight of about 100,000, and a sialoglycoprotein with a molecular weight of about 40,000. Cross-linking of purified NaK-ATPase with the (o-phenanthroline)2-cupric ion complex (CP) results in the reversible formation of dimers, trimers, tetramers, and pentamers of the large polypeptide and loss of NaK-ATPase activity. ATPase activity is partially recovered if NaK-ATPase is incubated with beta-mercaptoethanol after treatment with CP. In contrast to these results, if NaK-ATPase is cross-linked in crude canine kidney microsomes, only a dimer of the large polypeptide is formed. No cross-linking of the sialoglycoprotein to the large polypeptide is detected when NaK-ATPase is cross-linked in purified form. However, when NaK-ATPase is reacted with CP in either purified or microsomal form, the sialoglycoprotein cross-links to itself yielding a high molecular weight aggregate. The results show that the functional subunit structure of NaK-ATPase consists of at least two large polypeptides.