αβ spectrin coiled coil association at the tetramerization site
On the basis of sequence homology studies, it has been suggested that the association of human erythrocytes α and β spectrin at the tetramerization site involves interactions between helices. However, no empirical details are available, presumably due to the experimental difficulties in studying spectrin molecules because of its size and/or its structural flexibility. It has been speculated that erythrocyte tetramerization involves helical bundling rather than coiled coil association. We have used recombinant spectrin peptides to model α and β spectrin to study their association at the tetramerization site. Two α peptides, Spα1-156 and Spα1-368, and one β peptide, Spβ1898-2083, were used as model peptides to demonstrate the formation of the αβ complex. We also found that the replacement of R28 in Spα1-368 to give Spα1-368R28C abolished complex formation with the β peptide. Circular dichroism techniques were used to monitor the secondary structures of the individual peptides and of the complex, and the results showed that both Spα1-156 and Spβ1898-2083 peptides in solution, separately, included helices that were not paired with other helices in the absence of their binding partners. However, in a mixture of Spα1-156 and Spβ1898-2083 and formation of the αβ complex, the unpaired helices associated to form coiled coils. Since the sequences of these two peptides that are involved in the coiled coil association are derived from a native protein, the information obtained from this study also provides insight toward a better understanding of naturally occurring coiled coil subunit-subunit association.