*Article* **Pressure and Chemical Unfolding of an** α**-Helical Bundle Protein: The GH2 Domain of the Protein Adaptor GIPC1**

**Cécile Dubois 1, Vicente J. Planelles-Herrero <sup>2</sup> , Camille Tillatte-Tripodi 1, Stéphane Delbecq 1, Léa Mammri <sup>1</sup> , Elena M. Sirkia 2, Virginie Ropars 2, Christian Roumestand 1,\* and Philippe Barthe <sup>1</sup>**


**Abstract:** When combined with NMR spectroscopy, high hydrostatic pressure is an alternative perturbation method used to destabilize globular proteins that has proven to be particularly well suited for exploring the unfolding energy landscape of small single-domain proteins. To date, investigations of the unfolding landscape of all-β or mixed-α/β protein scaffolds are well documented, whereas such data are lacking for all-α protein domains. Here we report the NMR study of the unfolding pathways of GIPC1-GH2, a small α-helical bundle domain made of four antiparallel α-helices. Highpressure perturbation was combined with NMR spectroscopy to unravel the unfolding landscape at three different temperatures. The results were compared to those obtained from classical chemical denaturation. Whatever the perturbation used, the loss of secondary and tertiary contacts within the protein scaffold is almost simultaneous. The unfolding transition appeared very cooperative when using high pressure at high temperature, as was the case for chemical denaturation, whereas it was found more progressive at low temperature, suggesting the existence of a complex folding pathway.

**Keywords:** protein folding; NMR; high hydrostatic pressure; thermodynamic stability; α-helical bundle
