Regulated unfolding: a basic principle of intraprotein signaling in modular proteins

• Most signaling proteins are multidomain proteins.
• Sensory domains and output domains are structurally separated.
• Localized unfolding of a bistable linker is the key to signaling.
• Structural features favor evolution of modularized proteins.

Modular proteins possess N-terminal sensor domains connected with different C-terminal output domains. Different output domains, for example, phosphodiesterases adenylyl cyclases, are regulated by identical N-terminal domains. Therefore, the mechanisms of intraprotein signaling share properties suitable to regulation of disparate output enzymes, which see the same signal but react differently. The common denominator is a reversible switch of folding/unfolding that connects sensor and output domains. In the inhibited state, output domains are restrained, whereas in the activated state domains are released to assemble according to intrinsic domain properties. We review recent work investigating the mechanism of intraprotein signaling and discuss how this signaling mechanism may have contributed to the evolutionary diversity of specific small molecule-binding domains without loss of regulatory properties.