Filaments protofilaments

With regard to microtubular ultrastructure, micro filaments (5-7 run in diameter) are composed of filamentous actin. The tubule-like structures are formed by a, P-tubulin heterodimers. The wall is composed of 13 parallel protofilaments. Various microtubule-associated proteins and motor proteins (kinesin and dynein) are bound to the wall. The microtubule is a polar structure, i.e., plus and minus ends. 

Fig. 4. New structural models for amyloid and prion filaments with the parallel and in-register arrangement of //-strands in the //-sheets. //-Strands are denoted by arrows. The filaments are formed by hydrogen-bonded stacks of repetitive units. Axial projections of single repetitive units corresponding to each model are shown on the top. Lateral views of the overall structures are on the bottom. (A) The core of a //-helical model of the //-amyloid protofilament (Petkova et al., 2002). Two such protofilaments coil around one another to form a //-amyloid fibril. (B) The core of a //-helical model of the HET-s prion fibril (Ritter et al., 2005). The repetitive unit consists of two //-helical coils. (C) The core of a superpleated //-structura l model suggested for yeast prion Ure2p protofilaments and other amyloids (Kajava et al., 2004).

In many amyloid systems, filament polymorphism has been observed by EM (e.g., Goldsbury et al., 1997, 2000). Structural variations may be expressed in terms of long-range axial repeats (Goldsbury et al, 2005 Jimenez et al., 2001), diameter (Louis et al., 2005), and/or number of protofilaments (Jimenez et al., 2002). Solid-state NMR has also been used to detect slight structural differences in Alzheimer s /1-peptide filaments... 

In the keratins, large parts of the peptide chain show right-handed a-helical coiling. Two chains each form a left-handed superhelix, as is also seen in myosin (see p. 65). The superhelical keratin dimers join to form tetramers, and these aggregate further to form protofilaments, with a diameter of 3 nm. Finally, eight protofilaments then form an intermediate filament, with a diameter of 10 nm (see p.204). 

The components of the intermediate filaments belong to five related protein families. They are specific for particular cell types. Typical representatives include the cytokeratins, desmin, vimentin, glial fibrillary acidic protein (GFAP), and neurofilament. These proteins all have a rod-shaped basic structure in the center, which is known as a superhelix ( coiled coil see keratin, p. 70). The dimers are arranged in an antiparallel fashion to form tet-ramers. A staggered head-to-head arrangement produces protofilaments. Eight protofilaments ultimately form an intermediary filament. 

In both case (gel and dried gels) the advantage or a scheme based on the protcfilament is the high degree of resultant structural flexibility. Interf ila.ment contacts of the physical reticulation can occur by filament juxtaposition, by a fusion of all protofilaments of the two incoming filaments (say n protofilaments) to form an n-start helical fusion zone of reticulation or by an int erf ilament exchange of just one or iP.ore protofilaments. 

Lashuel HA, LaBrenz SR, Woo L, SerpeU LC, Kelly JW. Protofilaments, filaments, ribbons and fibrils from peptidomimetic self-assembly implications for amyloid fibril formation and materials science. J Am Chem Soc 2000 122 5262-5277. 

FIGURE 4-11 Structure of hair, (a) Hair a-keratin is an elongated a helix with somewhat thicker elements near the amino and carboxyl termini. Pairs of these helices are interwound in a left-handed sense to form two-chain coiled coils. These then combine in higher-order structures called protofilaments and protofibrils. About four protofibrils—32 strands of a-keratin altogether—combine to form an intermediate filament. The individual two-chain coiled coils in the various substructures also appear to be interwound, but the handedness of the interwinding and other structural details are unknown, (b) A hair is an array of many a-keratin filaments, made up of the substructures shown in (a). 

A common architecture of intermediate filaments is a staggered head-to-tail and side-by-side association of pairs of the coiled-coil dimers into 2- to 3-nm protofilaments and further association of about eight protofilaments to form the 10-nm intermediate filaments.286 290 296... 

Figure 7-31 A model for the structure of keratin microfibrils of intermediate filaments. (A) A coiled-coil dimer, 45-nm in length. The helical segments of the rod domains are interrupted by three linker regions. The conformations of the head and tail domains are unknown but are thought to be flexible. (B) Probable organization of a protofilament, involving staggered antiparallel rows of dimers. From Jeffrey A. Cohlberg297...

Lashuel, H.A., LaBrenz, S.R., Woo, L., Serpell, L.C., and Kelly, J.W. "Protofilaments, filaments, ribbons, and fibrils from peptidomimetic self-assembly Implications for amyloid fibril formation and materials science". J. Am. Chem. Soc. 122(22), 5262-5277 (2000). 

Figure 2.25. Structure of keratin protofibrils. The diagram illustrates the structure of keratin in intermediate filaments containing a helical sequences. Two coils are wound around each other and then packed into protofilaments. Eight protofilaments are packed into a filament.

Two unfortunate names that can easily lead to confusion have been given to the in-pathway conformations protofibrils and protofilaments. Protofibrils are, similar to oligomers, mostly unstructured but linear (31). However, in most cases the term protofibrils is used interchangeably with the term oligomer. The name protofilaments refers to linear aggregates with P-sheet structure that have a diameter of 3nm and are usually 50-100 nm in length. Mature, elongated protofilaments, also called filaments, intertwine to form protein fibrils. Fibrils... 

Using myosin manipulation or scanning probe methods, these ATP hydrolysis coupled steps have been resolved into multiple stochastic substeps (Fig. 12.Id). The laser trap method, however, has not been able to resolve substeps. We attribute this to the large scanning probe slowing the substeps to a time within our system s time resolution. The size of the substeps was 5.5 nm, corresponding to the interval of actin monomers on a protofilament in a two-stranded filament. Substeps occurred both in the forward and back-... 

MW 50,000 each dimer binds two GTP, calcium may inhibit polymerization polymerizes in protofilaments which bond in parallel to form a cylinder of 13 filaments each dimer interacts with 4 or 6 adjacent dimers dimer can be added or lost at either end encoded by a multigene family. 

Ip W, Heuser JE. Subunit structure of desmin and vimentin protofilaments and how they assemble into intermediate filaments. Ann NY Acad Sci. 1985 455 185-199. 

Filament (four right-hand twisted protofilaments)... 

The a-helical rodlike domains of two keratin polypeptides form a coiled coil. Two staggered antiparallel rows of these dimers form a supercoiled protofilament. Hundreds of filaments, each containing four protofilaments, form a macrofibril. 

The cytosol of a eukaryotic cell contains three types of filaments that can be distinguished on the bases of their diameter, type of subunit, and subunit arrangment (Figure 5-29). Actin filaments, also called microfilaments, are 8-9 nm in diameter and have a twisted two-stranded structure. Microtubules are hollow tubelike structures, 24 nm in diameter, whose walls are formed by adjacent protofilaments. Intermediate filaments (IFs) have the structure of a 10-nm-dlameter rope. 

---