Protein Unfolding and Aggregation near a Hydrophobic Interface

The behavior of proteins near interfaces is relevant for biological and medical purposes. Previous results in bulk show that, when the protein concentration increases, the proteins unfold and, at higher concentrations, aggregate. Here, we study how the presence of a hydrophobic surface affects this course of events. To this goal, we use a coarse-grained model of proteins and study by simulations their folding and aggregation near an ideal hydrophobic surface in an aqueous environment by changing parameters such as temperature and hydrophobic strength, related, e.g., to ions concentration. We show that the hydrophobic surface, as well as the other parameters, affect both the protein unfolding and aggregation. We discuss the interpretation of these results and define future lines for further analysis, with their possible implications in neurodegenerative diseases.

Crystal structure of caspase-11 CARD provides insights into caspase-11 activation

Murine caspase-11 is the centerpiece of the non-canonical inflammasome pathway that can respond to intracellular LPS and induce pyroptosis. Caspase-11 contains two components, an N-terminal caspase recruitment domain (CARD) and a C-terminal catalytic domain. The aggregation of caspase-11 is thought to promote the auto-processing and activation of caspase-11. However, the activation mechanism of caspase-11 remains unclear. In this study, we purified the caspase-11 CARD fused to an MBP tag and found it tetramerizes in solution. Crystallographic analysis reveals an extensive hydrophobic interface formed by the H1–2 helix mediating homotypic CARD interactions. Importantly, mutations of the helix H1–2 hydrophobic residues abolished the tetramerization of MBP-tagged CARD in solution and failed to induce pyroptosis in cells. Our study provides the first evidence of the homotypic interaction mode for an inflammatory caspase by crystal model. This finding demonstrates that the tetramerization of the N-terminal CARD can promote releasing of the catalytic domain auto-inhibition, leading to the caspase-11 activation.

The HIV-1 Vpu transmembrane domain topology and formation of a hydrophobic interface with BST-2 are critical for Vpu-mediated BST-2 downregulation

Viruses belonging to the M group of human immunodeficiency virus (HIV-1) are the most virulent among the four HIV-1 groups. One factor that distinguishes the M group HIV-1 from others is Vpu, a membrane localized accessory protein, which promotes the release of virions by neutralizing the antiviral host cell protein BST-2. To investigate if this activity is determined by the topology of Vpu or by conserved amino acid residues, we prepared chimeric forms of Vpu by replacing its transmembrane domain with those from its topological homologs. Although the chimeric Vpu proteins downregulated BST-2, these substantially reduced virus production as well. Molecular modeling studies on Vpu from different HIV-1 groups and the chimeric Vpu proteins showed that shape and the availability of a hydrophobic interface are more important for BST-2 antagonism than conservation of the amino acid sequence. Our data suggest that the HIV-1 Vpu-M protein has evolved topologically to interact with BST-2, and that the Vpu/BST-2 interface can be exploited as a target to limit HIV-1 replication.