The RAF cysteine-rich domain: Structure, function, and role in disease.

RAF kinases, consisting of ARAF, BRAF and CRAF, are direct effectors of RAS GTPases and critical for signal transduction through the RAS-MAPK pathway. Driver mutations in BRAF are commonplace in human cancer, while germline mutations in BRAF and CRAF cause RASopathy development syndromes. However, there remains a lack of effective drugs that target RAF function, which is partially due to the complexity of the RAF activation cycle. Therefore, greater understanding of RAF regulation is required to identify new approaches that target its function in disease. A key piece of this puzzle is the RAF zinc finger, often referred to as the cysteine-rich domain (CRD). The CRD is a lipid and protein binding domain which plays complex and opposing roles in the RAF activation cycle. Firstly, it supports the RAS-RAF interaction during RAF activation by binding to phosphatidylserine (PS) in the plasma membrane and by making direct RAS contacts. Conversely, under quiescent conditions the CRD also plays a critical role in maintaining RAF in a closed, autoinhibited state. However, the interplay between these activities and their relative importance for RAF activation were not well understood. Recent structural and biochemical studies have contributed greatly to our understanding of these roles and identified functional differences between BRAF CRD and that of CRAF. This chapter provides an in-depth review of the CRDs roles in RAF regulation and how they may inform novel approaches to target RAF function.