The seven-fold circularized propellar is called a b propellar by the authors.

From the name, it is clear that seven b sheets are needed to give the b subunit of the G protein the correct shape for its function.



You should note that the terms
"b sheet" and
"b subunit"
share b in their names only by coincidence.

The subunit that carries the propellar could have been called the "a subunit" but by chance it was called the "b subunit".

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