Chaperones 3 strands

D. Specific Chaperones Involved in the Folding of Triple p-Stranded Folds... 

A peptide of some 150 residues of GroEL (e.g., residues 191-345) folds stably into a monomer that is functionally active as a chaperone in vitro 97 Further, it can be covalently attached to agarose and other solid supports where the monodispersed fragment is extremely active as a chaperone.98 Crystal structures of recombinant minichaperones reveal that the active site is a flexible hydrophobic patch.99 It fits best to extended /3 strands. The basic function of GroEL is to provide a surface for binding exposed hydrophobic patches of denatured... 

Fig. 3. Periplasmic pilus chaperone consensus sequence. Amino acid sequence of the PapD chaperone (top line) and consensus sequence derived from the comparison of twelve chaperones (second line). Amino acids are indicated using the one-letter code. In the consensus sequence, a letter shows a residue that is present in at least eight out of twelve sequences, an asterisk designates an invariant residue, and a box shows a position with a hydrophobic residue in all twelve periplasmic chaperones. The arrows underneath the sequence represent the /3 strands found in the PapD structure.

Chromatin assembly is coupled with DNA replication. Immediately after DNA replication, the parental histones from the original chromosome are randomly divided between the two daughter strands of DNA (14). The remaining complements of histones are assembled from newly synthesized histones. Newly synthesized histones H3 and H4 are acetylated, and these acetyl marks are removed after their incorporation into DNA (15). Assembly of chromatin requires the functions of histone chaperones and ATP-dependent chromatin remodeling factors (16). 

Histone chaperones bind histones and facilitate their proper deposition onto DNA by preventing nonspecific histone-DNA interactions (17). Two major histone chaperones are CAF-1 and NAP-1. CAF-1 localizes to the replication fork by binding PCNA and facilitates the deposition of histones H3 and H4 onto the newly synthesized DNA strands (18,19). Subsequently, NAP-1 facilitates the deposition of histones H2A and H2B to complete the nucleosome (20). Using in vitro nucleosome assembly and nuclease digestion mapping assays, it was shown that the periodic spacing of nucleosomes requires the function of ATP-dependent chromatin remodeling factors, such as the ACF/ISWI complex (16, 21). 

Fig. 2 Schematic illustration of chaperone/usher-assisted assembly of type 1 fimbriae. Subunits (A, F, G, H) enter the periplasm via the Sec system and transiently remain associated with the inner membrane (step 1). FimC chaperone (C) binds newly translocated, partially unfolded subunits to form soluble and stable chaperone subunit complexes (step 2). Targeting of the binary FimC FimH pre-assembly complex to an empty FimD usher (D) initiates assembly. At the usher, an incoming chaperone subunit complex attacks the chaperone subunit complex capping the base of the growing fibre (step 3). The usher catalyses DSE in which the capping chaperone at the base is released, the N-terminal Gd donor strand of the attacking subunit is inserted into the polymerization cleft of the subunit at the base to form a new fibre module, and a new chaperone-capped subunit is added at the base. For further details of the assembly process, see text...

The subunits of fimbriae are constructed essentially as Ig-like P-sandwiches, but with a circular permutation that positions the sequence corresponding to the seventh, C-terminal, Ig P-strand (strand G of a canonical Ig domain) at the N-terminus of the polypeptide sequence [32-35] (Figs. 3a and 4a). In a typical Ig fold, the top edge of the sandwich, defined by the A and F strands, is capped by the C-terminal G strand, which is hydrogen bonded to the F strand and provides hydrophobic residues to the core of the fold. Free pilin subunits are only marginally stable, and no structure for a monomeric pilin in the absence of chaperone has been obtained. Sometimes, in the absence of the chaperone, soluble, domain-swapped pilin dimers [36] or trimers [37, 38] are formed and have been reported in their crystal structures. However, the oligomerization of pilins in this manner is a dead-end process, and the pilins in these oligomers are not able to assemble into fibrillar structures. 

Fig. 3 Schematic illustration of a relation between the Ig and the pilin fold b DSC before (subunit bound to chaperone below) and after (in fibre module above) DSE. The ellipsoids represent the P-sandwich of the Ig/pilin fold as viewed down at the AF edge of the sandwich the rectangles represent P sheets and strands (as labelled) viewed edge on...

Fig. 4 Pilin structure a ribbon diagram of a Cafl subunit illustrating the Ig-like pilin fold. Main secondary structural elements are labelled Nte denotes the N-terminal extension (disordered unless used for DSC of a neighbouring subunit in a fibre) b surface representation of Cafl illustrating the hydrophobic acceptor cleft (hydrophobic residues Ala, Val, Leu, lie, Phe, Met coloured yellow) with the Gd strand from the CaflM chaperone (stick model) bound sub-pockets of the acceptor cleft, designated P1-P5 in the nomenclature of Sauer et al. [34], are labeHed...

Fig. 5 Chaperone structure. Ribbon diagram of FimC chaperone from the structure of the FimC FimH complex [32], p-strands in the N-terminal domain are labelled. The hydrophobic residues in the G, donor strand are shown as stick models and labelled. Also shown are the two...

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