The structure of tussah silk fibroin (with a note on the structure of β-poly- L -alanine)

The fiber properties of lepidopteran silk depend on the amino acid repeats that interact during H-fibroin polymerization. The aim of our research was to relate repeat composition to insect biology and fiber strength. Representative regions of the H-fibroin genes were sequenced and analyzed in three pyralid species: wax moth (Galleria mellonella), European flour moth (Ephestia kuehniella), and Indian meal moth (Plodia interpunctella). The amino acid repeats are species-specific, evidently a diversification of an ancestral region of 43 residues, and include three types of regularly dispersed motifs: modifications of GSSAASAA sequence, stretches of tripeptides GXZ where X and Z represent bulky residues, and sequences similar to PVIVIEE. No concatenations of GX dipeptide or alanine, which are typical for Bombyx silkworms and Antheraea silk moths, respectively, were found. Despite different repeat structure, the silks of G. mellonella and E. kuehniella exhibit similar tensile strength as the Bombyx and Antheraea silks. We suggest that in these latter two species, variations in the repeat length obstruct repeat alignment, but sufficiently long stretches of iterated residues get superposed to interact. In the pyralid H-fibroins, interactions of the widely separated and diverse motifs depend on the precision of repeat matching; silk is strong in G. mellonella and E. kuehniella, with 2-3 types of long homogenous repeats, and nearly 10 times weaker in P. interpunctella, with seven types of shorter erratic repeats. The high proportion of large amino acids in the H-fibroin of pyralids has probably evolved in connection with the spinning habit of caterpillars that live in protective silk tubes and spin continuously, enlarging the tubes on one end and partly devouring the other one. The silk serves as a depot of energetically rich and essential amino acids that may be scarce in the diet.

supporting

confidence: 76%

Correlation between Fibroin Amino Acid Sequence and Physical Silk Properties

Fedic

Z̆urovec

Sehnal

2003

“…This dimension corresponds to the length of two peptide bonds and indicates that ␤-strands run parallel to the fiber axis. A similar orientation is found in amyloid fibrils of a peptide from PrP c (55) and in fibers of natural silk (56,57). Despite the fact that in our experiments no magnetic field was used to orient the samples, we still acquire preferred orientation because of the capillary.…”

Section: Glycation Of Albumin Induces Formation Of Toxic Aggre-mentioning

confidence: 68%

Glycation Induces Formation of Amyloid Cross-β Structure in Albumin

Bouma¹,

Kroon-Batenburg²,

Wu³

et al. 2003

265

177

Amyloid fibrils are components of proteinaceous plaques that are associated with conformational diseases such as Alzheimer's disease, transmissible spongiform encephalopathies, and familial amyloidosis. Amyloid polypeptides share a specific quarternary structure element known as cross-␤ structure. Commonly, fibrillar aggregates are modified by advanced glycation end products (AGE). In addition, AGE formation itself induces protein aggregation. Both amyloid proteins and protein-AGE adducts bind multiligand receptors, such as receptor for AGE, CD36, and scavenger receptors A and B type I, and the serine protease tissue-type plasminogen activator (tPA). Based on these observations, we hypothesized that glycation induces refolding of globular proteins, accompanied by formation of cross-␤ structure. Using transmission electron microscopy, we demonstrate here that glycated albumin condensates into fibrous or amorphous aggregates. These aggregates bind to amyloid-specific dyes Congo red and thioflavin T and to tPA. In contrast to globular albumin, glycated albumin contains amino acid residues in ␤-sheet conformation, as measured with circular dichroism spectropolarimetry. Moreover, it displays cross-␤ structure, as determined with x-ray fiber diffraction. We conclude that glycation induces refolding of initially globular albumin into amyloid fibrils comprising cross-␤ structure. This would explain how glycated ligands and amyloid ligands can bind to the same multiligand "cross-␤ structure" receptors and to tPA.

“…Five structural silk categories were recognized on the basis of x-ray diffraction studies and amino acid analyses (13). The silk of B. mori was classified as group 1, which is characterized by dense molecule packing (intersheet distance 9.3 Å) corresponding to high glycine content (14), whereas the tussah silk of Antheraea pernyi was classified as group 3 with 10.6 Å intersheet packing that is consistent with high alanine content (15). These data were recently supported by full elucidation of the primary H-fibroin structures.…”

mentioning

confidence: 99%

Unique Molecular Architecture of Silk Fibroin in the Waxmoth,Galleria mellonella

Z̆urovec

Sehnal

2002

Proteins of silk fibers are characterized by reiterations of amino acid repeats. Physical properties of the fiber are determined by the amino acid composition, the complexity of repetitive units, and arrangement of these units into higher order arrays. Except for very short motifs of 6 -10 residues, the length of repetitive units and the number of these units concatenated in higher order assemblies vary in all spider and lepidopteran silks analyzed so far. This paper describes an exceptional silk protein represented by the 500-kDa heavy chain fibroin (H-fibroin) of the waxmoth, Galleria mellonella. Its non-repetitive N-terminal (175 residues) and C-terminal (60 residues) parts, the overall gene organization, and the nucleotide sequence around the TATA box show that it is homologous to the H-fibroins of other Lepidoptera. However, over 95% of the protein consists of highly ordered repetitive structures that are unmatched in other species. The repetitive region includes 11 assemblies AB 1 AB 1 AB 1 AB 2 (AB 2 )AB 2 of remarkably conserved polypeptide repeats A (63 amino acid residues), B 1 (43 residues), and B 2 (18 residues). The repeats contain a high proportion of Gly (31.6%), Ala (23.8%), Ser (18.1%), and of residues with long hydrophobic side chains (16% for Leu, Ile, and Val combined). The presence of the GLGGLG and SSAASAA(AA) motifs suggests formation of pleated ␤-sheets and their stacking into crystallites. Conspicuous conservation of the apolar sequence VIVI followed by DD or ED is interpreted as indicating the importance of hydrophobicity and electrostatic charge in H-fibroin cross-linking. The environment of G. mellonella larvae within bee cultures requires continuous production of silk that must be both strong and elastic. The spectacular arrangement of the repetitive H-fibroin region apparently evolved to meet these requirements.