Computer-based database searching and protein multiple sequence alignment has identified a novel clan of zinc metallopeptidases, which, by phylogenetic analysis, has been shown to contain six subfamilies. The family is characterized by four common transmembrane segments and three conserved sequence motifs. The combination of topology analysis and motif identification has detected three potential Zn2+ coordinating residues. Only two of the sequences of this novel zinc metallopeptidase clan possess any functional annotation, one of which is able to cleave its substrate within a cytosol/transmembrane segment junction. A number of observations suggest that the remaining members of this novel clan may also cleave their substrates within transmembrane segments.

Proteolytic studies have enabled two of the three putative domains of the fibrinolytic protein streptokinase to be isolated and characterized (Conejero-Lara F et al., 1996, Protein Sci 5:2583–2591). The N-terminal domain, however, could not be isolated in these experiments because of its susceptibility to proteolytic cleavage. To complete the biophysical characterization of the domain structure of streptokinase we have overexpressed, purified, and characterized the N-terminal region of the protein, residues 1–146. The results show this is cooperatively folded with secondary structure content and overall stability closely similar to those of the equivalent region in the intact protein.

Bacteriorhodopsin (bR) and halorhodopsin (hR) found in the cell membrane of halobacteria are the simplest known light energy-converting systems. They are small proteins and require only light and a lipid bilayer to generate an electrochemical ion gradient. In overall structure, chromophore, and photoreactions, they closely resemble the visual pigments of animals, but those are signal, not energy, transducers and no obvious sequence homologies originally indicated an evolutionary connection. Signal-transducing archaebacterial rhodopsins have also been detected, but they are closely related to the energy transducers. These pigments show the same seven-helix structural motif common to energy-transducing bacterial rhodopsins, visual pigments, and other G-protein-linked sensors. Nevertheless, their sensory function is not mediated by either ion transport or G-proteins, but resembles the chemosensory transduction system of eubacteria. This review, however, will trace only the development of a mechanistic model for the active transport function of bR up to 1995.

This book brings together in one nicely produced volume a timely, well-written, and comprehensive collection of articles by specialists in the field. The book should be useful for those already working in related areas as well as for newcomers. Where it is particularly helpful is in condensing the vast literature on ubiquitin and putting it into perspective. The easy-to-read style of the best chapters should make the book accessible to new students in the field, and for the more experienced reader it provides a useful source of reference. Many of the chapters are excellent, and the style is mostly clear and concise.