Conformation of the Polyalanine Repeats in Minor Ampullate Gland Silk of the Spider Nephila clavipes

Abstract

Solid state 13C NMR is used to identify the conformation of alanine residues in minor ampullate gland silk from Nephila clavipes and in a genetically engineered protein based on the consensus sequence of MaSp2, a protein present in low concentrations in major ampullate gland silk. The results of the NMR on minor ampullate gland silk are compared to previous NMR data on major ampullate gland silk, and the results on the genetically engineered protein are compared to previous NMR data on the gland fibroin from the major ampullate gland. Differences in the secondary structure of major and minor ampullate gland silk are correlated with mechanical properties. The alanine residues of major ampullate gland silk have previously been shown to consist almost entirely of β-sheet conformations. In contrast, the conformations of the alanine residues of minor ampullate gland silk are more heterogeneous, with a larger fraction of the alanine residues in non-β-sheet conformations. It is hypothesized that these non-β-sheet structures in minor ampullate gland silk are part of the cause for its lower tensile strength. In addition, both unprocessed genetically engineered silk and lyophilized major ampullate gland fibroin, have similar alanine Cβ carbon chemical shifts yet differ in their alanine Cα carbon chemical shifts. This suggests that this genetically engineered protein is good starting material for biofiber synthesis but that it does not exactly mimic the gland fibroin.