Elucidation of intermediate (mobile) and slow (solidlike) protein motions in bovine lens homogenates by carbon-13 NMR spectroscopy.

The motional dynamics of lens cytoplasmic proteins present in calf lens homogenates were investigated by two 13C nuclear magnetic resonance (NMR) techniques sensitive to molecular motion to further define the organizational differences between the cortex and nucleus. For the study of intermediate (mobile) protein rotational reorientation motion time scales [rotational correlation time (tau 0) range of 1-500 ns], we employed 13C off-resonance rotating frame spin-lattice relaxation, whereas for the study of slow (solidlike) motions (tau 0 greater than or equal to 10 microseconds) we used the solid-state NMR techniques of dipolar decoupling and cross-polarization. The frequency dependence of the peptide bond carbonyl off-resonance rotating frame spectral intensity ratio of the lens proteins present in native calf nuclear homogenate (42% protein) at 35 degrees C indicates the presence of a polydisperse mobile protein fraction with a tau 0,eff (mean) value of 57 ns. This mean value is consistent with the average value calculated from the known water-soluble nuclear lens protein polydispersity assuming a cytoplasmic viscosity 3 times that of pure water. Lowering the temperature to 1 degree C, a temperature which produces the cold cataract, results in an overall decrease in tau 0,eff to 43 ns, suggesting a selective removal of beta H-, LM-, and possibly gamma s-crystallins from the mobile lens protein population. The presence of solidlike or motionally restricted protein species was established by dipolar decoupling and cross-polarization. The fraction of motionally restricted protein in the nuclear region varied from 0.35 to 0.45 in the temperature range of 35-1 degree C. For native cortical homogenate (25% protein), the off-resonances rotating frame spectral intensity ratio frequency-dependent curves for the protein carbonyl resonance yielded tau 0,eff values of 34 and 80 ns at 35 and 1 degree C, respectively. Both values were reconciled with the known lens cortex soluble protein polydispersity using an assumed cytoplasmic viscosity 1.5 times that of pure water at the same temperature. Comparison of proton dipolar-decoupled and nondecoupled 13C NMR spectra of native cortical homogenate at 20 degrees C indicates the absence of significant contributions from slowly tumbling, motionally restricted species. This interpretation was confirmed by the failure to detect significant lens protein 13C-1H cross-polarization at this temperature. However, at 1 degree C, the fraction of solidlike protein was 0.15. Concentrated cortical homogenates at 20 degrees C (42% protein), by contrast, gave cross-polarization spectra with maximum absolute signal intensities 50-70% of native nuclear homogenates, but with similar magnetization parameters...