Analysis of the functions of the signal peptidase complex in the midgut of Tribolium castaneum.

Signal peptidase complexes (SPCs) are conserved from bacteria to human beings, and are typically composed of four to five subunits. There are four genes encoding SPC proteins in the red flour beetle, Tribolium castaneum. To understand their importance to insect development, double-stranded RNA for each SPC gene was injected into red flour beetles at the early larval and adult stages. Knockdown of all four signal peptidase genes was lethal to larvae. Moreover, larvae had difficulty with old cuticle ecdysis. Knockdown of TcSPC12 alone did not affect pupal or adult development. When TcSPC12, TcSPC18, and TcSPC25 were knocked down in larvae, the melanization of hemocytes and midguts was observed. When knocked down in larvae and adults, TcSPC18 induced severe cell apoptosis in midguts, and the adult midgut lost the ability to maintain crypts after knockdown of TcSPC18, indicating its importance to midgut cell proliferation and differentiation. Knockdown of TcSPC22 or TcSPC25 also resulted in many apoptotic cells in the midguts. However, TcSPC12 appeared to be unimportant for midgut development. We conclude that TcSPC18 is essential for maintaining the adult midgut crypts.