A LysM Domain Intervenes in Sequential Protein-Protein and Protein-Peptidoglycan Interactions Important for Spore Coat Assembly in Bacillus subtilis.

At a late stage in spore development in Bacillus subtilis, the mother cell directs synthesis of a layer of peptidoglycan known as the cortex between the two forespore membranes, as well as the assembly of a protective protein coat at the surface of the forespore outer membrane. SafA, the key determinant of inner coat assembly, is first recruited to the surface of the developing spore and then encases the spore under the control of the morphogenetic protein SpoVID. SafA has a LysM peptidoglycan-binding domain, SafALysM, and localizes to the cortex-coat interface in mature spores. SafALysM is followed by a region, A, required for an interaction with SpoVID and encasement. We now show that residues D10 and N30 in SafALysM, while involved in the interaction with peptidoglycan, are also required for the interaction with SpoVID and encasement. We further show that single alanine substitutions on residues S11, L12, and I39 of SafALysM that strongly impair binding to purified cortex peptidoglycan affect a later stage in the localization of SafA that is also dependent on the activity of SpoVE, a transglycosylase required for cortex formation. The assembly of SafA thus involves sequential protein-protein and protein-peptidoglycan interactions, mediated by the LysM domain, which are required first for encasement then for the final localization of the protein in mature spores.IMPORTANCEBacillus subtilis spores are encased in a multiprotein coat that surrounds an underlying peptidoglycan layer, the cortex. How the connection between the two layers is enforced is not well established. Here, we elucidate the role of the peptidoglycan-binding LysM domain, present in two proteins, SafA and SpoVID, that govern the localization of additional proteins to the coat. We found that SafALysM is a protein-protein interaction module during the early stages of coat assembly and a cortex-binding module at late stages in morphogenesis, with the cortex-binding function promoting a tight connection between the cortex and the coat. In contrast, SpoVIDLysM functions only as a protein-protein interaction domain that targets SpoVID to the spore surface at the onset of coat assembly.

The coat morphogenetic protein SpoVID is necessary for spore encasement in Bacillus subtilis.

Endospores formed by Bacillus subtilis are encased in a tough protein shell known as the coat, which consists of at least 70 different proteins. We investigated the process of spore coat morphogenesis using a library of 40 coat proteins fused to green fluorescent protein and demonstrate that two successive steps can be distinguished in coat assembly. The first step, initial localization of proteins to the spore surface, is dependent on the coat morphogenetic proteins SpoIVA and SpoVM. The second step, spore encasement, requires a third protein, SpoVID. We show that in spoVID mutant cells, most coat proteins assembled into a cap at one side of the developing spore but failed to migrate around and encase it. We also found that SpoIVA directly interacts with SpoVID. A domain analysis revealed that the N-terminus of SpoVID is required for encasement and is a structural homologue of a virion protein, whereas the C-terminus is necessary for the interaction with SpoIVA. Thus, SpoVM, SpoIVA and SpoVID are recruited to the spore surface in a concerted manner and form a tripartite machine that drives coat formation and spore encasement.

Herpesvirus capsid association with the nuclear pore complex and viral DNA release involve the nucleoporin CAN/Nup214 and the capsid protein pUL25.

After penetrating the host cell, the herpesvirus capsid is transported to the nucleus along the microtubule network and docks to the nuclear pore complex before releasing the viral DNA into the nucleus. The viral and cellular interactions involved in the docking process are poorly characterized. However, the minor capsid protein pUL25 has recently been reported to be involved in viral DNA uncoating. Here we show that herpes simplex virus type 1 (HSV-1) capsids interact with the nucleoporin CAN/Nup214 in infected cells and that RNA silencing of CAN/Nup214 delays the onset of viral DNA replication in the nucleus. We also show that pUL25 interacts with CAN/Nup214 and another nucleoporin, hCG1, and binds to the pUL36 and pUL6 proteins, two other components of the herpesvirus particle that are known to be important for the initiation of infection and viral DNA release. These results identify CAN/Nup214 as being a nuclear receptor for the herpesvirus capsid and pUL25 as being an interface between incoming capsids and the nuclear pore complex and as being a triggering element for viral DNA release into the nucleus.

Structural framework for DNA translocation via the viral portal protein.

Tailed bacteriophages and herpesviruses load their capsids with DNA through a tunnel formed by the portal protein assembly. Here we describe the X-ray structure of the bacteriophage SPP1 portal protein in its isolated 13-subunit form and the pseudoatomic structure of a 12-subunit assembly. The first defines the DNA-interacting segments (tunnel loops) that pack tightly against each other forming the most constricted part of the tunnel; the second shows that the functional dodecameric state must induce variability in the loop positions. Structural observations together with geometrical constraints dictate that in the portal-DNA complex, the loops form an undulating belt that fits and tightly embraces the helical DNA, suggesting that DNA translocation is accompanied by a 'mexican wave' of positional and conformational changes propagating sequentially along this belt.

Interaction between coat morphogenetic proteins SafA and SpoVID.

Morphogenetic proteins such as SpoVID and SafA govern assembly of the Bacillus subtilis endospore coat by guiding the various protein structural components to the surface of the developing spore. Previously, a screen for peptides able to interact with SpoVID led to the identification of a PYYH motif present in the C-terminal half of the SafA protein and to the subsequent demonstration that SpoVID and SafA directly interact. spoVID and safA spores show deficiencies in coat assembly and are lysozyme susceptible. Both proteins, orthologs of which are found in all Bacillus species, have LysM domains for peptidoglycan binding and localize to the cortex-coat interface. Here, we show that the interaction between SafA and SpoVID involves the PYYH motif (region B) but also a 13-amino-acid region (region A) just downstream of the N-terminal LysM domain of SafA. We show that deletion of region B does not block the interaction of SafA with SpoVID, nor does it bring about spore susceptibility to lysozyme. Nevertheless, it appears to reduce the interaction and affects the complex. In contrast, lesions in region A impaired the interaction of SafA with SpoVID in vitro and, while not affecting the accumulation of SafA in vivo, interfered with the localization of SafA around the developing spore, causing aberrant assembly of the coat and lysozyme sensitivity. A peptide corresponding to region A interacts with SpoVID, suggesting that residues within this region directly contact SpoVID. Since region A is highly conserved among SafA orthologs, this motif may be an important determinant of coat assembly in the group of Bacillus spore formers.