Mechanically stressed biological materials like tendon, spider silk or mussel byssal threads are typically composite materials comprising multi-domain proteins, in which molecular building blocks contribute to overall material function. Mussel byssal threads are the anchorage of sessile mytilid mussels, which withstand recurring external loads from waves and tides. A single thread is elastic and ductile proximally, while the distal portion exhibits an extraordinary stiffness and toughness with a transient gradient of both mechanical features along the thread. The main components of byssal threads include a set of various collagen-like structural proteins (preCols) consisting of a collagenous core sequence flanked by globular domains. Here, structural analysis using polarized Fourier-transform infrared spectroscopy (FTIR) on stretched distal portions of mussel byssal threads determines the impact of external linear load on various molecular moieties. It is concluded that the preCol collagenous core domain is the main load-bearing element in distal byssal threads, while polyalanine beta-sheets in the flanking domains, similar to those found in spider silk proteins, provide high stiffness at low strains. Load dissipation is mediated by domain stretching of amorphous glycine-rich helical moieties followed by complete unfolding of the preCol flanking domains. (C) 2011 Elsevier Inc. All rights reserved.