THE MULTICATALYTIC PROTEINASE COMPLEX (PROTEASOME) - STRUCTURE AND CONFORMATIONAL-CHANGES ASSOCIATED WITH CHANGES IN PROTEOLYTIC ACTIVITY

The multicatalytic proteinase complex or proteasome is a high-molecula r-mass multisubunit proteinase which is found in the nucleus and cytop lasm of eukaryotic cells. Electron microscopy of negatively stained ra t liver proteinase preparations suggests that the particle has a hollo w cylindrical shape (approximate width 11 nm and height 17 nm using me thylamine tungstate as the negative stain) with a pseudo-helical arran gement of subunits rather than the directly stacked arrangement sugges ted previously. The side-on view has a 2-fold rotational symmetry, whi le end-on there appears to be six or seven subunits around the ring. T his model is very different from that proposed by others for the prote inase from rat liver but resembles the structure of the simpler archae bacterial proteasome. The possibility of conformational changes associ ated with the addition of effectors of proteolytic activity has been i nvestigated by sedimentation velocity analysis and dynamic light-scatt ering measurements. The results provide the first direct evidence for conformational changes associated with the observed positive co-operat ivity in one component of the peptidylglutamylpeptide hydrolase activi ty as well as with the stimulation of peptidylglutamylpeptide hydrolas e activities by MnCl2. In the latter case, there appears to be a corre lation between changes in the shape of the molecule and the effect on activity. KCl and low concentrations of SDS may also act by inducing c onformational changes within the complex. Sedimentation-velocity measu rements also provide evidence for the formation of intermediates durin g dissociation of the complex by urea, guanidinium chloride or sodium thiocyanate. Dissociation of the complex either by these agents or by treatment at low pH leads to inactivation of its proteolytic component s. The results suggest that activation and inhibition of the various p roteolytic activities may be mediated by measurable changes in size an d shape of the molecules.