Untangling spider silk evolution with spidroin terminal domains.

Area of Science:

• Evolutionary biology
• Biochemistry
• Genomics

Background:

• Spidroins are large structural proteins forming spider silk.
• Repetitive spidroin sequences hinder evolutionary analysis; non-repetitive C-terminal domains offer limited data.
• N-terminal domains, though difficult to obtain, hold key phylogenetic information.

Purpose of the Study:

• To evaluate the functional significance and phylogenetic utility of spidroin N-terminal domains.
• To reconstruct the evolutionary diversification of spider silks using N-terminal domain data.
• To investigate the evolutionary history of spidroin structural motifs.

Main Methods:

• Sequencing approximately 1,900 silk gland cDNAs from nine diverse spider species.
• Phylogenetic analyses of N-terminal domains, C-terminal domains, and combined datasets.
• Comparative analysis of sequence identity, predicted secondary structure, and hydrophobicity of N-terminal domains.

Main Results:

• 11 novel spidroin N-termini were identified, showing high sequence, structural, and hydrophobic similarity.
• Combined phylogenetic analysis of N- and C-termini yielded the most resolved evolutionary trees.
• Spidroin paralogs generally grouped by silk gland type, with exceptions for major ampullate spidroins.
• Key structural motifs for tensile strength evolved early; motifs for extensibility evolved convergently.

Conclusions:

• The conserved N-terminal domain is a universal spidroin feature, likely present since silk production began.
• N-terminal domains play a crucial role in spider silk assembly.
• The spidroin gene family has undergone significant radiation, including duplications and expression pattern shifts, leading to diverse silk properties.