For many glycoproteins, the carbohydrate groups confer important physical properties such as conformational stability, protease resistance, charge and water-binding capacity. Equally important, however, are the roles of carbohydrate groups in biological recognition, where sequence diversity provides signals for protein targeting and cell-cell interactions.
The role of cyclic AMP (cAMP) in the regulation of mammalian cell proliferation has been the subject of controversy. Negative control was demonstrated in the 1970s, but evidence of positive control in other cell types has been neglected. Recent evidence which demonstrates such a control in the yeast Saccharomyces cerevisiae has now made this concept acceptable.
Many polypeptides can self-assemble into functional structures while others assemble only in the presence of additional proteins (molecular chaperones) which are not components of the final structure. We discuss here the effect that the recognition of the essential roles played by these proteins in assembly processes may have on the principle of spontaneous self-assembly.
Phosphatidylcholine turnover cycles can generate second messengers (including diacylglycerol and arachidonic acid). Guanine nucleotide-binding proteins are implicated in the coupling of agonist receptors to the activation of phospholipases A sub(2), C and D. In many instances, these agonists also stimulate the resynthesis of phosphatidylcholine via activation of CTP:phosphocholine cytidylyltransferase.
The light-driven water-splitting/oxygen-evolving enzyme remains one of the great enigmas of plant biology. However, due to the recent expansion of research efforts on this enzyme, it is grudgingly giving up some of its secrets.
Gene expression can be controlled by changing the half-lifes of specific mRNAs in response to intracellular events or to extracellular stimuli. Many experiments suggest that mRNA turnover in mammalian cells is linked to poly(A) metabolism, which might be affected by the efficiency of the cytoplasmic poly(A) binding protein in protecting poly(A) from nucleolytic attach.
Three models for the initiation of protein folding are considered in the light of recent experiments. The three models emphasize the possible roles of: (1) a hydrophobic collapse, (2) formation of secondary structure, or (3) formation of one or more specific interactions. Whether these models are likely to be contradictory or complementary is discussed.
Proteolytic processing is a common and effective mechanism of physiological regulation. The basic principle is a conformational change induced in the protein precursor by the post-translational proteolytic cleavage of a specific peptide bond. The extension of earlier studies of model zymogens to more complex systems of physiological regulation, using methods of both protein chemistry and molecular biology, has enormously extended knowledge of the repertoire of proteolytic processing reactions and has contributed significantly to current studies of the structure, domain organization and evolution of proteins.
The transport of inorganic and organic ions across the plasma membrane and organelle membranes of higher plants by ion channels, electrogenic pumps and co-transporters is essential to vital processes such as osmoregulation, growth, development, signal transduction and the storage of solutes. Recent studies have led to the identification of specialized transport proteins in the plasma membrane and vacuolar membrane of higher plant cells. Here the authors have integrated the functional aspects of these membrane proteins into a model which proposes a novel basis for ion transport processes involved in the regulation of gas exchange in leaves.
The protein tyrosine phosphatases comprises a family of enzymes that specifically dephosphorylate tyrosyl residues. Determination of the amino acid sequence of a major low molecular mass form isolated from human placenta (PTPase 1B) provided the basis for the first identification of transmembrane proteins that bear intracellular phosphatase domains. The existence of such molecules, bearing the hallmarks of receptors, raises the exciting possibility of a novel mechanism of signal transduction in which the early events involve the ligand-induced dephosphorylation of tyrosyl residues in proteins.
Bacterial plasmid resistance systems that maintain low intracellular levels of toxic heavy metals by pumping the substrates out as rapidly as they accumulate sometimes work at the biochemical level as efflux ATPases. The two systems responsible for arsenic and cadmium resistance have recently been sequenced. Comparison of the deduced amino acid sequences with those of better characterized ATPases has revealed certain structural and sequence similarities.