Cyclostreptin and microtubules: is a low-affinity binding site required?
Cyclostreptin (CS) is a recently discovered natural product with cytotoxic activity caused by microtubule stabilization. It is the only known microtubule-stabilizing agent (MSA) that covalently binds to tubulin. It also exhibits the fast-binding kinetics seen for other MSAs. Through careful peptide digestion and mass spectrometry analysis, Buey et al. found that two amino acids are labeled by CS: Asn228, near the known taxane-binding site, and Thr220, in the type I microtubule pore. This led Buey et al. to propose Thr220 resides at the site previously predicted to be a way station or low-affinity site. By using molecular dynamics simulations and structural considerations of the microtubule pore and tubulin dimer, we conclude that postulation of a low-affinity site is unnecessary to explain the available experimental data. An alternative explanation views the microtubule pore as a structural entity that presents a substantial kinetic barrier to ligand passage to the known taxane-binding site-an entry point to the microtubule lumen that becomes completely blocked if cyclostreptin is bound at Thr220. Simulations of the free dimer also suggest a common mechanism of microtubule stabilization for taxane site MSAs through their conformational effect on the M-loop. Such an effect explains the low tubulin polymerization caused by cyclostreptin in vitro despite its covalent attachment.
Prussia
Andrew J
AJ
Department of Chemistry, Emory University, 1515 Dickey Drive, Atlanta, GA 30322, USA.
Yang
Yutao
Y
Geballe
Matthew T
MT
Snyder
James P
JP
D001665:Binding Sites; D001952:Bridged-Ring Compounds / Q000737:chemistry; D034261:Epothilones / Q000737:chemistry; D005079:Excipients / Q000737:chemistry / Q000378:metabolism; D008870:Microtubules / Q000737:chemistry* / Q000378:metabolism; D056004:Molecular Dynamics Simulation; D017239:Paclitaxel / Q000737:chemistry; D011083:Polycyclic Compounds / Q000737:chemistry* / Q000378:metabolism; D043823:Taxoids / Q000737:chemistry; D014404:Tubulin / Q000737:chemistry / Q000378:metabolism
D001952:Bridged-Ring Compounds; D034261:Epothilones; D005079:Excipients; C408303:FR 182877; D011083:Polycyclic Compounds; D043823:Taxoids; D014404:Tubulin; C080625:taxane; D017239:Paclitaxel