The mechanism of specific binding of free cholesterol by the steroidogenic acute regulatory protein: evidence for a role of the C-terminal α-helix in the gating of the binding site. Issue 2 (21st January 2009)

• Record Type: Journal Article
• Authors: Roostaee, Alireza; Barbar, Élie; Lavigne, Pierre; Lehoux, Jean-Guy
• Is Part Of: Bioscience reports. Volume 29:Issue 2(2009)
• Page Start: 89
• Page End: 101
• Publication Date: 2009-01-21
• Subjects: binding -- CD -- lipoid congenital adrenal hyperplasia (LCAH) -- steroidogenesis -- steroidogenic acute regulatory protein (StAR) -- structure; Molecular biology -- Periodicals; Cytology -- Periodicals; 572.8
• DOI: 10.1042/BSR20080111
• Languages: English
• ISSNs: 0144-8463

Abstract

Abstract : Steroidogenesis depends on the delivery of free cholesterol to the inner mitochondrial membrane by StAR (steroidogenic acute regulatory protein). Mutations in the StAR gene leads to proteins with limited cholesterol-binding capacity. This gives rise to the accumulation of cytoplasmic cholesterol, a deficit in steroid hormone production and to the medical condition of lipoid congenital adrenal hyperplasia. A detailed understanding of the mechanism of the specific binding of free cholesterol by StAR would be a critical asset in understanding the molecular origin of this disease. Previous studies have led to the proposal that the C-terminal α-helix 4 of StAR was undergoing a folding/unfolding transition. This transition is thought to gate the cholesterol-binding site. Moreover, a conserved salt bridge (Glu 169 –Arg 188 ) in the cholesterol-binding site is also proposed to be critical to the binding process. Interestingly, some of the documented clinical mutations occur at this salt bridge (E169G, E169K and R188C) and in the C-terminal α-helix 4 (L275P). In the present study, using rationalized mutagenesis, activity assays, CD, thermodynamic studies and molecular modelling, we characterized the α-helix 4 mutations L271N and L275P, as well as the salt bridge double mutant E169M/R188M. The results provide experimental validation for the gating mechanism of the cholesterol-binding site by the C-terminal α-helix and the importance of the salt bridge in the binding mechanism. Altogether, our results offer a molecular framework for understanding the impact of clinical mutations on the reduction of the binding affinity of StAR for free cholesterol.