The capture spiral of a spider's orb web is made of flagelliform silk, providing high elasticity and an outstanding toughness, perfectly suited for trapping prey. Flagelliform silk comprises mainly one single protein (FLAG) with an estimated molecular weight of 360 kDa. We engineered constructs mimicking distinct domains of FLAG (eFLAG) and produced them recombinantly to analyze the structure-function relationship of FLAG domains and assembly properties of FLAG. While in solution the small carboxy-terminal domain is structured, domains from the repetitive core region adopt a conformation typical for intrinsically unstructured proteins. To investigate the influence of the respective domains on solubility and assembly, we tested the aggregation behaviour of individual domains and domain ensembles in presence of conditions known to trigger silk assembly. Both, the length of the repetitive core domain as well as the presence of the carboxy-terminal non-repetitive domain showed impact on eFLAG aggregation. (C) 2009 Elsevier Inc. All rights reserved.