Assembly of multimeric proteins. Effect of mutations in the alpha-subunit on membrane assembly and activity of pyridine nucleotide transhydrogenase.

The pyridine nucleotide transhydrogenase of Escherichia coli is an inner membrane protein of two different subunits (alpha and beta). It functions as a proton pump. The highly hydrophilic carboxy-terminal tail of ten amino acid residues in the alpha-subunit determines the correct folding and proper assembly of the beta-subunit leading to a functional enzyme. Premature termination of the alpha-subunit six amino acid residues from the carboxy-terminal end abolishes the activity completely. Although the two subunits are still assembled into the membrane, the conformation of the beta-subunit is perturbed. Systematic truncation and site-directed substitutions revealed that at least one positive charge in the carboxy-terminal region is required for efficient assembly of the two subunits to give a functional enzyme, while a phenylalanine residue, essential for activity, has no apparent effect on the extent of assembly of the two subunits.