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Abstract:
The superfamily of b-propeller proteins is characterized by an enormous diversity at the sequence level. However, all proteins share a common structural feature: they build their globular structure from repetitive units, the propeller blades. In the 50-odd structures of b-propellers in the Protein Data Bank, the number of blades ranges from four to eight. It is remarkable that nature can use such diverse numbers of structurally similar repeats to construct compact protein structures. How did this protein fold arise? One possibility is that single blades oligomer- ized and were only later combined to a single polypeptide chain. To evaluate this scenario, we tried to construct full-sized, oligo- meric propellers from a single blade. This blade was derived from the consensus over a naturally occurring protein in which the individual blades are very similar to each other. We multiplied this blade to obtain larger building blocks and used these in assembly reactions. We determined the oligomeric status of the resulting complexes by gel-size exclusion chromatography and static light scattering. Using various building blocks in varying stoichiometry, we constructed oligomeric propellers with different numbers of blades. We also determined the molecular structure of a single blade in the context of such a propeller by an NMR spectroscopy. We conclude that b-propellers can be created by the oligomerization of smaller pieces. Depending on the number of blades (even/uneven) in each building block, protein oligomers of different size result. This indicates that b-propeller proteins may have evolved from single blades.
