  We investigate the folding transition of a single diblock copolymer
consisting of a semiflexible and a flexible block. We obtain a {\it
Saturn-shaped} core-shell conformation in the folded state, in which the
flexible block forms a core and the semiflexible block wraps around it. We
demonstrate two distinctive features of the core-shell structures: (i) The
kinetics of the folding transition in the copolymer are significantly more
efficient than those of a semiflexible homopolymer. (ii) The core-shell
structure does not depend on the transition pathway.
