A minimal sequence code for switching protein structure and function

摘要

We present here a structural and mechanistic description of how a protein changes its fold and function, mutation by mutation. Our approach was to create 2 proteins that (ⅰ) are stably folded into 2 different folds, (ⅱ) have 2 different functions, and (ⅲ) are very similar in sequence. In this simplified sequence space we explore the muta-tional path from one fold to another. We show that an IgG-binding, 4β+α fold can be transformed into an albumin-binding, 3-α fold via a mutational pathway in which neither function nor native structure is completely lost. The stabilities of all mutants along the pathway are evaluated, key high-resolution structures are determined by NMR, and an explanation of the switching mechanism is provided. We show that the conformational switch from 4β+α to 3-α structure can occur via a single amino acid substitution. On one side of the switch point, the 4β+α fold is >90% populated (pH 7.2, 20 ℃). A single mutation switches the conformation to the 3-a fold, which is >90% populated (pH 7.2, 20 ℃). We further show that a bifunctional protein exists at the switch point with affinity for both IgG and albumin.