Chorion, the major component of silkmoth eggshell, consists of the A and
B classes of low-molecular weight structural proteins. Chorion protects
the oocyte and the developing embryo from environmental hazards and this
is due to the extraordinary physical and chemical properties of its
constituent proteins. We have shown previously [FEBS Lett. 479 (2000)
141; 499 (2001) 268] that peptide-analogues of the A and B classes of
chorion proteins form amyloid fibrils under a variety of conditions,
which led us to propose that silkmoth chorion is a natural, protective
amyloid. In this work, we present data showing conclusively that, the
first main step of amyloid-like fibrillogenesis of chorion peptides is
the formation of nuclei of liquid crystalline nature, which is
reminiscent of spider-silk formation. We show that these
liquid-crystalline nuclei (spherulites) ‘collapse’/deteriorate to form
amyloid fibrils in a spectacular manner, important, it seems, for
chorion morphogenesis and amyloid fibrillogenesis in general. The
molecular ‘switch’ causing this spectacular transformation is, most
probably, a conformational transition to the structure of chorion
peptides, from a left-handed parallel beta-helix to an
antiparallel-beta-pleated sheet. Apparently, these peptides were
suitably designed to play this role, after millions of years of
molecular evolution. (C) 2003 Elsevier Inc. All rights reserved