Distinct contributions of model MaSp1 and MaSp2 like peptides to the mechanical properties of synthetic major ampullate silk fibers as revealed in silico

Abstract

All characterized major ampullate silks from orb-web weaving spiders are composites of primarily two different proteins: MaSp1 and MaSp2. The conserved association of MaSp1 and MaSp2 in these spider species, the highly conserved amino acid motifs, and variable ratios of MaSp1 to MaSp2 demonstrate the importance of both MaSp1 and MaSp2 to the strength and elasticity of the fiber. Computer simulated mechanical tests predicted differing roles for MaSp1 and MaSp2 in the mechanical properties of the fibers. Recombinant MaSp1 and MaSp2 proteins were blended and spun into fibers mimicking the computer-simulated conditions. Mechanical testing verified the differing roles of MaSp1 and MaSp2.

Keywords:

• spider silk
• mechanical properties
• MaSp1
• MaSp2

Supplementary Material

Methods and Materials

All chemicals used were obtained from Sigma Aldrich (St. Louis, MO, USA) unless otherwise stated.

Protein expression

Each protein was produced from approximately 12L of E. coli bacterial culture. The culture was permitted to reach an OD₆₀₀ of approximately 1 upon which IPTG (isopropyl-beta-D-thiogalactopyranoside) (Biosynth AG, Switzerland) was added to a 1 mM final concentration. After induction, the culture was allowed to produce the synthetic silk protein for 2.5 hours at which time the cells were harvested by centrifugation at 5500 rpm for 20 minutes. Cell pellets were resuspended in a three to one weight to volume ratio of binding buffer (5 mM imidazole, 0.5 M NaCl, 20 mM Tris (Fisher Chemicals, Fair Lawn, NJ, USA) pH 7.9) and frozen at −80 °C until purification.

Purification

Purification was accomplished using the N-terminal histidine tags provided by the pET vector system. To purify the silk-like protein of interest, nickel affinity chromatography (Ni-NTA resin; Novagen, Gibbstown, NJ, USA) was utilized. The manufacturer’s protocol was followed with the following modifications for the specific protein. The cells were lysed chemically with 0.5% lysozyme, 0.01% DNase, and 2% deoxycholate acid sodium salt monohydrate (MP Biomedicals, Aurora, OH, USA). Additionally, to ensure complete lysis, the resuspended cells were also sonicated (Misonix Sonicator 3000) for approximately five minutes. The soluble and insoluble fractions were separated using centrifugation (9500 rpm for approximately 10 minutes). Protein in the soluble fraction was allowed to bind to the resin for approximately 30 minutes on an orbital shaker (S500, VWR, West Chester, PA, USA) and then purified. To compete with nonspecific interactions a 30 mM imidazole wash (30 mM imidazole [Fisher Chemicals, Fair Lawn, NJ, USA], 0.5 M NaCl, 20 mM Tris pH 7.9) followed by a 60 mM imidazole (60 mM imidazole, 0.5 M NaCl, 20 mM Tris pH 7.9) were used. The strip fraction (100 mM EDTA, 0.2 M Tris, 0.5 M NaCl) relied on the chelating affects of EDTA (Fisher Chemicals, Fair Lawn, NJ, USA) to elute the protein. This fraction was dialyzed against water using a stirred cell with a 10 K membrane (Millipore Amicon, Billerica, MA, USA) and subsequently lyophilized.

Results

Supplementary Table 1 Amino acid sequence for the MaSp1 and MaSp2 repeat units from Nephila clavipes used for both the synthetic constructs as well as the molecular dynamics simulations. All MaSp1 fibers were spun from a construct with 8 repeat units to produce a protein of approximately 55 kD; whereas all MaSp2 fibers were spun from a construct with 16 repeat units to produce a protein of approximately 67 kD. Accession numbers U20329, M92913.

Supplementary Table 2 Comparison of the values for several mechanical properties for natural versus synthetic silk fibers. Natural fibers outperform synthetic versions in every property considered by at least one order of magnitude.

Acknowledgments

The NIH, the NSF, and the Department of Defense supported this work. Amanda E Brooks and Shane R Nelson contributed equally to this work. The authors report no conflicts of interest in this work.