Peptide Bond Formation between Aminoacyl-Minihelices by a Scaffold Derived from the Peptidyl Transferase Center.

The peptidyl transferase center (PTC) in the ribosome is composed of two symmetrically arranged tRNA-like units that contribute to peptide bond formation. We prepared units of the PTC components with putative tRNA-like structure and attempted to obtain peptide bond formation between aminoacyl-minihelices (primordial tRNAs, the structures composed of a coaxial stack of the acceptor stem on the T-stem of tRNA). One of the components of the PTC, P1c2UGGU (74-mer), formed a dimer and a peptide bond was formed between two aminoacyl-minihelices tethered by the dimeric P1c2UGGU. Peptide synthesis depended on both the existence of the dimeric P1c2UGGU and the sequence complementarity between the ACCA-3' sequence of the minihelix. Thus, the tRNA-like structures derived from the PTC could have originated as a scaffold of aminoacyl-minihelices for peptide bond formation through an interaction of the CCA sequence of minihelices. Moreover, with the same origin, some would have evolved to constitute the present PTC of the ribosome, and others to function as present tRNAs.

Impaired actin filaments decrease cisplatin sensitivity via dysfunction of volume-sensitive Cl- channels in human epidermoid carcinoma cells.

Cisplatin is a widely used platinum-based anticancer drug in the chemotherapy of numerous human cancers. However, cancer cells acquire resistance to cisplatin. So far, functional loss of volume-sensitive outwardly rectifying (VSOR) Cl- channels has been reported to contribute to cisplatin resistance of cancer cells. Here, we analyzed protein expression patterns of human epidermoid carcinoma KB cells and its cisplatin-resistant KCP-4 cells. Intriguingly, KB cells exhibited higher ß-actin expression and clearer actin filaments than KCP-4 cells. The ß-actin knockdown in KB cells decreased VSOR Cl- currents and inhibited the regulatory volume decrease (RVD) process after cell swelling. Consistently, KB cells treated with cytochalasin D, which depolymerizes actin filaments, showed smaller VSOR Cl- currents and slower RVD. Cytochalasin D also inhibited cisplatin-triggered apoptosis in KB cells. These results suggest that the disruption of actin filaments cause the dysfunction of VSOR Cl- channels, which elicits resistance to cisplatin in human epidermoid carcinoma cells.