  Intermediate filaments are the least explored among the large cytoskeletal
elements. We show here that they display conformational anomalies in narrow
microfluidic channels. Their unusual behavior can be understood as the
consequence of a previously undetected, large scale helically curved
superstructure. Confinement in a channel orders the otherwise soft, strongly
fluctuating helical filaments and enhances their structural correlations,
giving rise to experimentally detectable, strongly oscillating tangent
correlation functions. We propose an explanation for the detected intrinsic
curving phenomenon - an elastic shape instability that we call autocoiling. The
mechanism involves self-induced filament buckling via a surface stress located
at the outside of the cross-section. The results agree with ultrastructural
findings and rationalize for the commonly observed looped intermediate filament
shapes.
