The ClpX heat shock protein of Escherichia coli is a member of the universally conserved Hsp100 family of proteins, and possesses a putative zinc finger motif of the C4 type []. This presumed zinc binding domain (ZBD) is found at the N terminus of the ClpX protein. ClpX is an ATPase which functions both as a substrate specificity component of the ClpXP protease and as a molecular chaperone. ZBD is a member of the treble clef zinc finger family, a motif known to facilitate protein-ligand, protein-DNA, and protein-protein interactions and forms a constitutive dimer that is essential for the degradation of some, but not all, ClpX substrates [].
Zinc finger (Znf) domains are relatively small protein motifs which contain multiple finger-like protrusions that make tandem contacts with their target molecule. Some of these domains bind zinc, but many do not; instead binding other metals such as iron, or no metal at all. For example, some family members form salt bridges to stabilise the finger-like folds. They were first identified as a DNA-binding motif in transcription factor TFIIIA from Xenopus laevis (African clawed frog), however they are now recognised to bind DNA, RNA, protein and/or lipid substrates [, , , , ]. Their binding properties depend on the amino acid sequence of the finger domains and of the linker between fingers, as well as on the higher-order structures and the number of fingers. Znf domains are often found in clusters, where fingers can have different binding specificities. There are many superfamilies of Znf motifs, varying in both sequence and structure. They display considerable versatility in binding modes, even between members of the same class (e.g. some bind DNA, others protein), suggesting that Znf motifs are stable scaffolds that have evolved specialised functions. For example, Znf-containing proteins function in gene transcription, translation, mRNA trafficking, cytoskeleton organisation, epithelial development, cell adhesion, protein folding, chromatin remodelling and zinc sensing, to name but a few []. Zinc-binding motifs are stable structures, and they rarely undergo conformational changes upon binding their target. The ClpX heat shock protein of Escherichia coli is a member of the universally conserved Hsp100 family of proteins, and possesses a putative zinc finger motif of the C4 type []. This presumed zinc binding domain (ZBD) is found at the N terminus of the ClpX protein. ClpX is an ATPase which functions both as a substrate specificity component of the ClpXP protease and as a molecular chaperone. ZBD is a member of the treble clef zinc finger family, a motif known to facilitate protein-ligand, protein-DNA, and protein-protein interactions and forms a constitutive dimer that is essential for the degradation of some, but not all, ClpX substrates [].
ClpX is a member of the HSP (heat-shock protein) 100 family. Gel filtration and electron microscopy showed that ClpX subunits associate to form a six-membered ring that is stabilised by binding of ATP or nonhydrolysable analogs of ATP []. It functions as an ATP-dependent []molecular chaperone and is the regulatory subunit of the ClpXP protease [].ClpXP is involved in DNA damage repair, stationary-phase gene expression, and ssrA-mediated protein quality control. To date more than 50 proteins include transcription factors, metabolic enzymes, and proteins involved in the starvation and oxidative stress responses have been identified as substrates []. The N-terminal domain of ClpX is a C4-type zinc binding domain (ZBD) involved in substrate recognition. ZBD forms a very stable dimer that is essential for promoting the degradation of some typical ClpXP substrates such as lO and MuA [].
ClpX is a member of the HSP (heat-shock protein) 100 family. Gel filtration and electron microscopy showed that ClpX subunits associate to form a six-membered ring that is stabilised by binding of ATP or nonhydrolysable analogs of ATP []. It functions as an ATP-dependent []molecular chaperone and is the regulatory subunit of the ClpXP protease [].ClpXP is involved in DNA damage repair, stationary-phase gene expression, and ssrA-mediated protein quality control. To date more than 50 proteins include transcription factors, metabolic enzymes, and proteins involved in the starvation and oxidative stress responses have been identified as substrates []. The N-terminal domain of ClpX is a C4-type zinc binding domain (ZBD) involved in substrate recognition. ZBD forms a very stable dimer that is essential for promoting the degradation of some typical ClpXP substrates such as lO and MuA []. This entry represents ClpX subunit from bacteria.
This family of proteins represent HslU, a bacterial clpX homologue, which is an ATPase and chaperone belonging to the AAA Clp/Hsp100 family and a component of the eubacterial proteasome. ATP-dependent protease complexes are present in all three kingdoms of life, where they rid the cell of misfolded or damaged proteins and control the level of certain regulatory proteins. They include the proteasome in Eukaryotes, Archaea, and Actinomycetales and the HslVU (ClpQY, ClpXP) complex in other eubacteria. Genes homologous to eubacterial HslV, , (ClpQ,) and HslU (ClpY, ClpX) have also been demonstrated in to be present in the genome of trypanosomatid protozoa. They are expressed as precursors, with a propeptidethat is removed to produce the active protease. The protease is probably located in the kinetoplast (mitochondrion). Phylogenetic analysis shows that HslV and HslU from trypanosomatids form a single clad with other eubacterial homologues [].
Competence is the ability of a cell to take up exogenous DNA from its environment, resulting in transformation. It is widespread among bacteria and is probably an important mechanism for the horizontal transfer of genes. DNA usually becomes available by the death and lysis of other cells. Competent bacteria use components of extracellular filaments called type 4 pili to create pores in their membranes and pull DNA through the pores into the cytoplasm. This process, including the development of competence and the expression of the uptake machinery, is regulated in response to cell-cell signalling and/or nutritional conditions [].This family consists of several bacterial ComK proteins. ComK of Bacillus subtilis is a positive autoregulatory protein occupying a central position in the competence-signal-transduction network. It positively regulates the transcription of late competence genes, which specify morphogenetic and structural proteins necessary for construction of the DNA-binding and uptake apparatus, as well as the transcription of comK itself [, ]. ComK specifically binds to the promoters of the genes that it affects. It has been found that ClpX plays an important role in the regulation of ComK at the post-transcriptional level [].
