SPECIFIC RECOGNITION OF COILED COILS BY INFRARED-SPECTROSCOPY - ANALYSIS OF THE 3 STRUCTURAL DOMAINS OF TYPE-III INTERMEDIATE FILAMENT PROTEINS

The central domain of cytoplasmic intermediate filament (IF) proteins from vertebrates contains some 310 residues and forms a double-strande d coiled coil (rod) with a length of about 46 nm. The flanking termina l domains show a high cell type specific variability both in sequence and in length. Using Fourier transform infrared (FTIR) spectroscopy we measured secondary structures of isolated domains of type III and IV IF proteins and of the soluble tetramers and the filaments formed by t ype III IF proteins. The amide I spectrum of the desmin rod is virtual ly identical to the spectra of other coiled-coil proteins such as trop omyosin and the myosin rod. All these double-stranded coiled coils rev eal spectra distinctly different from classical alpha-helical spectra. The spectrum of coiled coils is a triplet of approximately equally st rong bands. One band occurs at normal alpha-helix position, while the other two are found at lower wavenumbers. Theoretical aspects of these findings are discussed in the accompanying paper by W. C. Reisdorf an d S. Krimm [(1996) Biochemistry 35, 1383-1386]. The amino-terminal hea d domain of desmin has a multicomponent spectrum with major fractions of beta-sheet. The carboxy-terminal tail domains of desmin and the neu rofilament proteins L and H, the latter in the phosphorylated and in t he dephosphorylated forms, have very similar FTIR spectra, indicating mostly random structure. The spectrum of desmin type III protofilament s is very similar to the sum of the spectra of the three isolated doma ins. Polymerization into filaments seems to induce a small change in s econdary structure.