Neutral theory of molecular evolution

Developed by Motoo Kimura, who argued that most of the evolution of DNA sequences occurs by chance (genetic drift) rather than by natural selection. 

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R-Smads comprising: 

An N-terminal MH1 domain

A central proline-rich linker 

A C-terminal MH2 domain

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MH1 domain contains:

DNA binding segment Nuclear-localization signal

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Phosphorylation of two serines in Ser-X-Ser motif at the extreme C-terminus of the MH2 domain by the activated RI separates the two domains, exposing the NLS for binding to importins

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Simultaneously, two serines in each Smad2/3 phosphorylated by RI receptor kinase bind to phosphoserine-binding sites in the MH2 domains of a Smad4

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Inside the nucleus, R smad-co smad complex binds to nuclear transcription factor TFE3 to activate transcription of PAI-1 & other ECM proteins such as fibronectin & collagens

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Within the nucleus, R-Smads are modified by:

Phosphorylation of linker domains

Acetylation of MH1 domains

Dephosphorylation of the C-terminal serines by nuclear

phosphatases