Meta-analysis reveals materiomic relationships in major ampullate silk across the spider phylogeny

Abstract

Spider major ampullate (MA) silk, with its combination of strength and extensibility, outperforms any synthetic equivalents. There is thus much interest in understanding its underlying materiome. While the expression of the different silk proteins (spidroins) appears an integral component of silk performance, our understanding of the nature of the relationship between the spidroins, their constituent amino acids and MA silk mechanics is ambiguous. To provide clarity on these relationships across spider species, we performed a meta-analysis using phylogenetic comparative methods. These showed that glycine and proline, both of which are indicators of differential spidroin expression, had effects on MA silk mechanics across the phylogeny. We also found serine to correlate with silk mechanics, probably via its presence within the carboxyl and amino-terminal domains of the spidroins. From our analyses, we concluded that the spidroin expression shifts across the phylogeny from predominantly MaSp1 in the MA silks of ancestral spiders to predominantly MaSp2 in the more derived spiders' silks. This trend was accompanied by an enhanced ultimate strain and decreased Young's modulus in the silks. Our meta-analysis enabled us to decipher between real and apparent influences on MA silk properties, providing significant insights into spider silk and web coevolution and enhancing our capacity to create spider silk-like materials.

The common house spider, Parasteatoda tepidariorum, maintains silk gene expression on sub-optimal diet

“…The ratio of MaSp1:MaSp2 is modelled as an important determinant of dragline silk material properties [ 15 , 27 ]. We found the transcript ratio for MaSp1 : MaSp2 was very similar between major ampullate (mean = 1.60) and minor ampullate (mean = 1.61) glands ( Fig 3A and 3B ).…”

“…However, MaSp2 contains numerous glycine-proline-glycine (GPG) motifs which form β-turn spirals [ 6 , 7 , 10 – 12 ]. Proline reduces protein alignment when in high abundance in major ampullate silk [ 13 ], and may also be hydroxylated after translation [ 14 , 15 ], both of which should contribute to the elasticity of the major ampullate silk. Indeed, across 85 species, those with a higher percentage of the proline-containing MaSp2 relative to the proline-poor MaSp1 have more extensible silk [ 13 , 15 ].…”

“…Proline reduces protein alignment when in high abundance in major ampullate silk [ 13 ], and may also be hydroxylated after translation [ 14 , 15 ], both of which should contribute to the elasticity of the major ampullate silk. Indeed, across 85 species, those with a higher percentage of the proline-containing MaSp2 relative to the proline-poor MaSp1 have more extensible silk [ 13 , 15 ]. Therefore, the ratio of MaSp1 to MaSp2 is an important determinant of silk mechanical properties, namely extensibility and strength.…”

Correlation between protein secondary structure and mechanical performance for the ultra-tough dragline silk of Darwin's bark spider

“…The primary amino acid sequence dictates the secondary structure of proteins, which is expected to influence the mechanical properties. Meta-analysis using phylogenetic comparative tests performed on various species also showed that spidroin expression is found to be integral to the mechanical properties of the MA silk [20]. For example, alanine and glycine promote strength-inducing β-sheet formation in MaSp1 by stacking together to form nanocrystals, while the bulky hydroxy proline group disrupts β-sheet formation and instead promotes β-turns since the -OH group in the hydroxy proline stabilizes the amorphous region with H-bonding in the β-spirals [19,[21][22][23].…”

Basic Principles in the Design of Spider Silk Fibers

“…The origin of the singular properties of spider silk can be traced down to the essential characteristics of their constitutive biomolecules, that include genetic organization [ 32 ], amino acid selection [ 33 ] and arrangement of the amino acids into protein sequences [ 34 , 35 ]. Not surprisingly, the formation of the fibers do depend on the assembly of the proteins, following a master plan established at these three levels [ 36 ].…”