Self-assembly in the major ampullate gland of Nephila clavipes

We present a tentative interpretation of the origin of nematic liquid crystalline order exhibited by dragline silk fibroin solutions collected from the spider Nephila clavipes. Liquid crystallinity is thought to confer certain rheological properties on the fibroin solution which are exploited during the dragline spinning process. We show that the feasibility of liquid crystallinity under physiological conditions depends critically on parameters characterising the amino-acid sequence of the fibroin molecules. “Dragline” silk secreted by the major ampullate gland of the orb-weaver spider Nephila clavipes is one of the best-characterised natural silks, notable in respect of its singular mechanical properties [1]. The spinning process itself, however, i.e., the structural transition undergone by fibroins initially dispersed in the aqueous environment of the gland, culminating with the extrusion of water-insoluble fiber, remains relatively poorly understood [2]. Recent in vitro experiments [3] have suggested that the solution in the gland enters a nematic (or twisted nematic) liquid crystalline state, which might play a rheological role in the spinning process and hence affect the mechanical quality of the extruded fiber [4]. Interestingly, Gatesy et al. [5] have argued that, in the evolutionary context, mechanical performance of natural silks exerts a particularly acute selection pressure on the aminoacid sequence of the constituent fibroin molecules. A stabilizing source of selection pressure appears necessary to explain the remarkably long evolutionary timescale (∼125 million years) over which the characteristic repeated-motif sequences of orb-weaving spiders have remained essentially unchanged. In this regard, correspondence between sequence and the ability to form a liquid crystalline phase in the gland, affecting spinning conditions, and hence dragline quality, would present a relevant perspective. Our objective here is to tentatively establish such a connection, addressing at the molecular level how nematic order emerges in the gland, and is controlled by sequence-related parameters. We develop from the premise that the dragline fibroin solution belongs to a general class of fibrillizing globular protein systems of which hemoglobin S is a well-known example [6]. The globules of these systems assemble reversibly into supramolecular rod-like structures, which, beyond a critical axial ratio, undergo a nematic ordering transition. The details of the assembly process are as follows. We identify three generic sources of protein-protein interaction contributing to an effective binding potential between fibroins. The principal contribution derives from the free energy of transfer ∆F of nonpolar amino acid from a hydrophobic environment to water the so-called hydrophobic effect. According to data collated and interpreted by Dill et al. [7], ∆F has a magnitude of approximately 2 kcal/mol, increasing with temperature up to a maximum at 60-80C: