Novel receptor polypeptides, polynucleotides encoding the polypeptides, and related compositions and methods are disclosed. When the elongated element is confined to a straight state, the effective hinges transfer compressive forces from each segment to the next so that the elongated element can be pushed to advance it through a conduit. Based on the lists, the system maintains an archive, which contains a copy of one version of each listed document, and material from which the other versions can be reconstructed.