Luca Mollica

Intrinsic disorder in measles virus nucleocapsids

The genome of measles virus is encapsidated by multiple copies of the nucleoprotein (N), forming helical nucleocapsids of molecular mass approaching 150 Megadalton. The intrinsically disordered C-terminal domain of N (NTAIL) is essential for transcription and replication of the virus via interaction with the phosphoprotein P of the viral polymerase complex. The molecular recognition element (MoRE) of NTAIL that binds P is situated 90 amino acids from the folded RNA-binding domain (NCORE) of N, raising questions about the functional role of this disordered chain. Here we report the first in situ structural characterization of NTAIL in the context of the entire N-RNA capsid. Using nuclear magnetic resonance spectroscopy, small angle scattering, and electron microscopy, we demonstrate that NTAIL is highly flexible in intact nucleocapsids and that the MoRE is in transient interaction with NCORE. We present a model in which the first 50 disordered amino acids of NTAIL are conformationally restricted as the chain escapes to the outside of the nucleocapsid via the interstitial space between successive NCORE helical turns. The model provides a structural framework for understanding the role of NTAIL in the initiation of viral transcription and replication, placing the flexible MoRE close to the viral RNA and, thus, positioning the polymerase complex in its functional environment.

The solution structure of the first PHD finger of autoimmune regulator in complex with non-modified histone H3 tail reveals the antagonistic role of H3R2 methylation

Plant homeodomain (PHD) fingers are often present in chromatin-binding proteins and have been shown to bind histone H3 N-terminal tails. Mutations in the autoimmune regulator (AIRE) protein, which harbours two PHD fingers, cause a rare monogenic disease, autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED). AIRE activates the expression of tissue-specific antigens by directly binding through its first PHD finger (AIRE-PHD1) to histone H3 tails non-methylated at K4 (H3K4me0). Here, we present the solution structure of AIRE-PHD1 in complex with H3K4me0 peptide and show that AIRE-PHD1 is a highly specialized non-modified histone H3 tail reader, as post-translational modifications of the first 10 histone H3 residues reduce binding affinity. In particular, H3R2 dimethylation abrogates AIRE-PHD1 binding in vitro and reduces the in vivo activation of AIRE target genes in HEK293 cells. The observed antagonism by R2 methylation on AIRE-PHD1 binding is unique among the H3K4me0 histone readers and represents the first case of epigenetic negative cross-talk between non-methylated H3K4 and methylated H3R2. Collectively, our results point to a very specific histone code responsible for non-modified H3 tail recognition by AIRE-PHD1 and describe at atomic level one crucial step in the molecular mechanism responsible for antigen expression in the thymus.

Mapping the population of protein conformational energy sub-states from NMR dipolar couplings.

The precision with which X-ray crystallography and nuclear magnetic resonance (NMR) have provided structural models of biologically active and inactive conformations of countless proteins belies an easily overlooked dilemma. Proteins are inherently dynamic, exhibiting conformational freedom on timescales from picoseconds to seconds, implicating structural rearrangements that are essential for their biological function. Classical structural biology determines static models, that afford little insight into the underlying conformational equilibrium. The role that structural dynamics play in biological processes can only be understood by characterizing all thermally accessible protein conformations and their populations. NMR spectroscopy is uniquely sensitive to the presence of conformational dynamics in solution. Residual dipolar couplings (RDCs) measured in weakly aligned proteins, scalar couplings, and chemical shifts, probe motions occurring on timescales faster than 100 s of microseconds. These parameters therefore offer general tools to characterize protein motion on physiologically important timescales. A common approach to the dynamic interpretation of RDCs is to combine experimental restraint terms with a classical potential-energy force field to develop a conformational ensemble in agreement with experimental data. RDCs have also been exploited to characterize the conformational space sampled by the protein backbone either by fitting experimental data to determine angular excursions of internuclear bond vectors, or in comparison with different levels of accelerated molecular dynamics (AMD) to describe the most appropriate ensemble. Comparison of motions modeled using the Gaussian axial fluctuation (GAF) model, with ensembles derived from restraint-free AMD, demonstrated that such methods can provide a convergent description of protein motion.

Atomic-Resolution Structural Dynamics in Crystalline Proteins from NMR and Molecular Simulation

Solid-state NMR can provide atomic-resolution information about protein motions occurring on a vast range of time scales under similar conditions to those of X-ray diffraction studies and therefore offers a highly complementary approach to characterizing the dynamic fluctuations occurring in the crystal. We compare experimentally determined dynamic parameters, spin relaxation, chemical shifts, and dipolar couplings, to values calculated from a 200 ns MD simulation of protein GB1 in its crystalline form, providing insight into the nature of structural dynamics occurring within the crystalline lattice. This simulation allows us to test the accuracy of commonly applied procedures for the interpretation of experimental solid-state relaxation data in terms of dynamic modes and time scales. We discover that the potential complexity of relaxation-active motion can lead to significant under- or overestimation of dynamic amplitudes if different components are not taken into consideration.

AIRE-PHD fingers are structural hubs to maintain the integrity of chromatin-associated interactome

Mutations in autoimmune regulator (AIRE) gene cause autoimmune polyendocrinopathy candidiasis ectodermal dystrophy. AIRE is expressed in thymic medullary epithelial cells, where it promotes the expression of peripheral-tissue antigens to mediate deletional tolerance, thereby preventing self-reactivity. AIRE contains two plant homeodomains (PHDs) which are sites of pathological mutations. AIRE-PHD fingers are important for AIRE transcriptional activity and presumably play a crucial role in the formation of multimeric protein complexes at chromatin level which ultimately control immunological tolerance. As a step forward the understanding of AIRE-PHD fingers in normal and pathological conditions, we investigated their structure and used a proteomic SILAC approach to assess the impact of patient mutations targeting AIRE-PHD fingers. Importantly, both AIRE-PHD fingers are structurally independent and mutually non-interacting domains. In contrast to D297A and V301M on AIRE-PHD1, the C446G mutation on AIRE-PHD2 destroys the structural fold, thus causing aberrant AIRE localization and reduction of AIRE target genes activation. Moreover, mutations targeting AIRE-PHD1 affect the formation of a multimeric protein complex at chromatin level. Overall our results reveal the importance of AIRE-PHD domains in the interaction with chromatin-associated nuclear partners and gene regulation confirming the role of PHD fingers as versatile protein interaction hubs for multiple binding events.

Pilotin–secretin recognition in the type II secretion system of Klebsiella oxytoca

A crucial aspect of the functionality of bacterial type II secretion systems is the targeting and assembly of the outer membrane secretin. In the Klebsiella oxytoca type II secretion system, the lipoprotein PulS, a pilotin, targets secretin PulD monomers through the periplasm to the outer membrane. We present the crystal structure of PulS, an all‐helical bundle that is structurally distinct from proteins with similar functions. Replacement of valine at position 42 in a charged groove of PulS abolished complex formation between a non‐lipidated variant of PulS and a peptide corresponding to the unfolded region of PulD to which PulS binds (the S‐domain), in vitro, as well as PulS function in vivo. Substitutions of other residues in the groove also diminished the interaction with the S‐domain in vitro but exerted less marked effects in vivo. We propose that the interaction between PulS and the S‐domain is maintained through a structural adaptation of the two proteins that could be influenced by cis factors such as the fatty acyl groups on PulS, as well as periplasmic trans‐acting factors, which represents a possible paradigm for chaperone–target protein interactions.

Multi‐Timescale Conformational Dynamics of the SH3 Domain of CD2‐Associated Protein using NMR Spectroscopy and Accelerated Molecular Dynamics

A complete understanding of the relationship between biological activity and molecular conformation requires an understanding of the thermally accessible potential energy landscape. An extensive set of experimental NMR residual dipolar couplings (RDCs) has been used to determine the conformational behavior of CD2AP SH3C on multiple timescales, using the Gaussian Axial Fluctuation model, and comparison to restraint-free accelerated molecular dynamics simulation. These robust analyses provide a comprehensive description of conformational fluctuations on picosecond to millisecond timescales. While the β-sheets show negligible slow motions, larger amplitude slow dynamics are found in the n-SRC and RT loops that mediate physiological interactions.

Nephrocystin-1 Forms a Complex with Polycystin-1 via a Polyproline Motif/SH3 Domain Interaction and Regulates the Apoptotic Response in Mammals

Mutations in PKD1, the gene encoding for the receptor Polycystin-1 (PC-1), cause autosomal dominant polycystic kidney disease (ADPKD). The cytoplasmic C-terminus of PC-1 contains a coiled-coil domain that mediates an interaction with the PKD2 gene product, Polycystin-2 (PC-2). Here we identify a novel domain in the PC-1 C-terminal tail, a polyproline motif mediating an interaction with Src homology domain 3 (SH3). A screen for interactions using the PC-1 C-terminal tail identified the SH3 domain of nephrocystin-1 (NPHP1) as a potential binding partner of PC-1. NPHP1 is the product of a gene that is mutated in a different form of renal cystic disease, nephronophthisis (NPHP). We show that in vitro pull-down assays and NMR structural studies confirmed the interaction between the PC-1 polyproline motif and the NPHP1 SH3 domain. Furthermore, the two full-length proteins interact through these domains; using a recently generated model system allowing us to track endogenous PC-1, we confirm the interaction between the endogenous proteins. Finally, we show that NPHP1 trafficking to cilia does not require PC-1 and that PC-1 may require NPHP1 to regulate resistance to apoptosis, but not to regulate cell cycle progression. In line with this, we find high levels of apoptosis in renal specimens of NPHP patients. Our data uncover a link between two different ciliopathies, ADPKD and NPHP, supporting the notion that common pathogenetic defects, possibly involving de-regulated apoptosis, underlie renal cyst formation.

Mapping the Native Conformational Ensemble of Proteins from a Combination of Simulations and Experiments: New Insight into the src-SH3 Domain

The biological function of a protein is strongly tied to the ensemble of three-dimensional conformations populated at physiological temperature, and dynamically transforming into each other. Experimental techniques such as nuclear magnetic resonance spectroscopy (NMR) provide a wealth of structural and dynamical information, which, in combination with an accurate atomic-level computational modeling, can disclose the details of protein behavior. We here propose a fast and efficient protocol employing molecular dynamics (MD) simulations and NMR chemical shifts, which allows one to reconstruct the detailed conformational ensemble of small globular proteins. In the case of the well-studied src-SH3 domain, we are able to obtain new important insight including the existence of a helical state in the RT loop and a pathway for single-file water diffusion in and out of the core.

Interactions of the C2 domain of human factor V with a model membrane.

Activated coagulation Factor V is an important cofactor of the coagulation cascade that catalyzes the formation of the prothrombinase complex on the surface of membranes rich in phosphatidyl‐l‐serine (PS). Here we report molecular dynamics simulations of the two crystallographic structures (the open and closed conformations) of domain C2 of coagulation Factor V (FaVC2). The calculations were performed in water (1.5 ns for each conformation) and in the presence of a neutral phospholipid bilayer model (POPE; 10 ns for each conformation) in order to describe the dynamics of the free (plasma circulating) and membrane bound forms of FaVC2. Water simulations confirmed the hypothesis that the plasma circulating form is in the closed conformation. In contrast, the membrane simulations showed that both conformations are energetically compatible with membrane binding. We have investigated the mechanism, the dynamics, and the energetics of the binding process. Our data are consistent with published estimates of the immersion depth of the aromatic residues (W26 and W27), and with mutagenesis studies involving specific residues located on the spikes at the bottom of the FaVC2 structure. Electrostatic interactions between the phospholipid head groups and hydrophilic residues at the bottom of the structure play a key role in the binding process by creating a large number of hydrogen bonds that anchor the protein to the membrane. The simulations identified a stable phospholipid binding pocket reminiscent of a previously suggested PS interaction site. Our structural data could contribute to the design of potential inhibitors able to disrupt membrane association. Proteins 2006.