The plant cell wall degrading apparatus of anaerobic bacteria includes a large multienzyme complex termed the "cellulosome." The complex assembles through the interaction of enzyme-derived dockerin modules with the multiple cohesin modules of the noncatalytic scaffolding protein. Here we report the crystal structure of the Clostridium cellulolyticum cohesindockerin complex in two distinct orientations. The data show that the dockerin displays structural symmetry reflected by the presence of two essentially identical cohesin binding surfaces. In one binding mode, visualized through the A16S/L17T dockerin mutant, the C-terminal helix makes extensive interactions with its cohesin partner. In the other binding mode observed through the A47S/F48T dockerin variant, the dockerin is reoriented by 180°and interacts with the cohesin primarily through the N-terminal helix. Apolar interactions dominate cohesin-dockerin recognition that is centered around a hydrophobic pocket on the surface of the cohesin, formed by Leu-87 and Leu-89, which is occupied, in the two binding modes, by the dockerin residues Phe-19 and Leu-50, respectively. Despite the structural similarity between the C. cellulolyticum and Clostridium thermocellum cohesins and dockerins, there is no cross-specificity between the protein partners from the two organisms. The crystal structure of the C. cellulolyticum complex shows that organism-specific recognition between the protomers is dictated by apolar interactions primarily between only two residues, Leu-17 in the dockerin and the cohesin amino acid Ala-129. The biological significance of the plasticity in dockerin-cohesin recognition, observed here in C. cellulolyticum and reported previously in C. thermocellum, is discussed.

This paper describes the first material to show the well-known light-induced excited spin-state trapping (LIESST) effect, the metastable excited state of which relaxes at a temperature approaching its thermal spin-crossover. Cooling polycrystalline [FeL(2)][BF(4)](2).x H(2)O (L=2,6-bis[3-methylpyrazol-1-yl]pyridine; x=0-1/3) at 1 K min(-1) leads to a cooperative spin transition, taking place in two steps centered at 147 and 105 K, that is only 54 % complete by magnetic susceptibility. Annealing the sample at 100 K for 2 h results in a slow decrease in chi(M)T to zero, showing that the remainder of the spin-crossover can proceed, but is kinetically slow. The crystalline high- and fully low-spin phases of [FeL(2)][BF(4)](2).x H(2)O are isostructural (C2/c, Z=8), but the spin-crossover proceeds via a mixed-spin intermediate phase that has a triple unit cell (C2/c, Z=24). The water content of the crystals is slowly lost on exposure to air without causing decomposition. However, the high-spin/mixed-spin transition in the crystal proceeds at 110+/-20 K when x=1/3 and 155+/-5 K when x=0, which correspond to the two spin-crossover steps seen in the bulk material. The high-spin state of the compound is generated quantitatively by irradiation of the low-spin or the mixed-spin phase at 10 K, and in approximately 70 % yield by rapidly quenching the sample to 10 K. This metastable high-spin state relaxes back to the low-spin ground state at 87+/-1 K in one, not two, steps, and without passing through the intermediate phase. This implies that thermal spin-crossover and thermally activated high-spin-low-spin relaxation in this material become decoupled, thus avoiding the physical impossibility of T(LIESST) being greater than T(1/2).

The matrix protein (M) of respiratory syncytial virus (RSV), the prototype viral member of the Pneumovirinae (family Paramyxoviridae, order Mononegavirales), has been crystallized and the structure determined to a resolution of 1.6 Å. The structure comprises 2 compact ␤-rich domains connected by a relatively unstructured linker region. Due to the high degree of side-chain order in the structure, an extensive contiguous area of positive surface charge covering Ϸ600 Å 2 can be resolved. This unusually large patch of positive surface potential spans both domains and the linker, and provides a mechanism for driving the interaction of the protein with a negatively-charged membrane surface or other virion components such as the nucleocapsid. This patch is complemented by regions of high hydrophobicity and a striking planar arrangement of tyrosine residues encircling the C-terminal domain. Comparison of the RSV M sequence with other members of the Pneumovirinae shows that regions of divergence correspond to surface exposed loops in the M structure, with the majority of viral species-specific differences occurring in the N-terminal domain.CD ͉ crystal structure ͉ respiratory syncytial virus ͉ sequence alignment R espiratory syncytial virus (RSV) is the prototype member of the Pneumovirinae, a subfamily of the Paramyxoviridae (order Mononegavirales). Morphologically, the extracellular virion consists of a lipid bilayer envelope, within which are embedded 3 glycoproteins, 2 of which (F and G) are important in cell attachment and viral entry into target cells. The third, the SH protein, contributes to pathology in the host (1). Internally, virions contain helical nucleocapsids that consist of N protein tightly bound to the negative-sense nonsegmented genomic RNA. The nucleocapsid in turn is associated with components of the viral RNA-dependent RNA polymerase (L, P, M2-1, and M2-2 proteins), forming the holo-nucleocapsid (2-4). Between the holo-nucleocapsid and the outer envelope there is a layer of matrix protein (M), which is associated peripherally with the membrane (5). The other family members of the Mononegavirales (Rhabdoviridae, Filoviridae, and Bornaviridae) all subscribe to this basic arrangement of the virion, although the overall morphology can vary between the families. For example, Paramyxoviridae virions are pleiomorphic, whereas the Rhabdoviridae have a regular bullet shape structure, and the Filoviridae have a more filamentous shape. Extracellular RSV virions form by a budding process that occurs at the plasma membrane within specialized lipid domains (5, 6) and M appears to drive the final assembly process, which is the incorporation of the holo-nucleocapsid and initiation of the budding process (7,8). Before budding, there is a coordinated assembly of viral components; and it is evident that the glycoproteins and M proteins are important determinants of the location on the plasma membrane at which the virus buds (9). It is also possible that the interaction between M and the glycoproteins, possibly mediat...