Competency states

The efficient uptake of precursor proteins depends on their presentation in a translocation competent state. This is maintained in vivo by the specific interaction with a highly conserved group of proteins, the heat-shock or stress related proteins (hps70s). These act as molecular chaperones and interact with the proteins to maintain them in a correctly folded state, a process which is ATP dependent. 

Subsequent reassessments now took divergent routes. Agren (1979) used thermodynamic data largely drawn from Orr and Chipman (1967) and re-characterised the magnetic component of the a-phase with the Hillert and Jarl model (see Chapter 8). The concept of two competing states in the 7-phase was abandoned as Orr and... 

The simplest way to describe the equilibrium between various competing states is to use a Shottky model (Weiss 1963) where... 

During cell division, a replication-competent state is established at the replication start sites, the pre-RC. This contains the ORC, the MCM proteins and the cdc6 protein. Formation of the pre-RC in late M phase and in G1 phase licenses the chromatin for DNA replication. With entry into S phase, the MCM proteins and the Cdc6 protein dissociate from the start site. Their phosphorylation by an active S phase cyclin-CDK complex is responsible for the dissociation. 

DNA and competent cells are mixed and incubated on ice for 15-30 min to allow the DNA to be taken up. The mixture is then heat-shocked at 42 °C for 1-2 min, which induces DNA repair enzymes, allowing the cells to recover from the competent state and DNA uptake. Recovery is allowed to occur in growth medium at 37 °C before streaking on an agar plate with appropriate selection. The plates are incubated at 37 °C overnight if successful, several colonies are formed. 

A growing body of evidence indicates that in addition to a targeting element, membrane translocation also requires that the secretory protein assumes a loosely folded or translocation-competent state. The focus of this review is the role that SecB and other molecular chaperones play in sponsoring efficient protein secretion in E. coli. 

Throughout this review, terms such as unfolded, loosely folded, or export-competent state are bandied about (no doubt distressing some disciples of protein structure). These terms are not meant to imply a single definable conformation, or the absence of a specific secondary or tertiary structure. In fact, several lines of evidence indicate that a translocation-competent precursor can contain considerable structure. For example, efficient signal peptide function requires that the hydrophobic core assume an a-helical conformation (Jones et ai, 1990), and evidence has been presented that enzymatically active DHFR (Freudl et al., 1988) and a fully folded biotin acceptor domain (Reed and Cronan, 1991)—or conformers of these proteins that are in equilibrium with the native states—can be translocated across the cytoplasmic membrane of E. coli in vivo. 

After coat protein pen tamer formation, the < (>X174 internal scaffolding protein binds to the underside of the pentamer and induces a conformational change in the particle. This change inhibits premature aggregation, and produces an assembly-competent state. B-protein binding is both necessary and sufficient to allow future interactions with... 

Figure 1 Overview of the synaptic vesicle cycle, (a) Within the presynaptic terminal, synaptic vesicles are filled with neurotransmitter by the action of specific vesicular neurotransmitter transporters, (b) Neurotransmitter-filled vesicles translocate to the active-zone membrane where they undergo docking, (c) Docked vesicles transition to a release-competent state through a series of priming or prefusion reactions, (d) Invasion of an action potential into the presynaptic terminal and subsequent calcium influx induces rapid fusion of the synaptic vesicle membrane with the terminal membrane, which thereby releases the neurotransmitter into the synaptic cleft, (e) Spent vesicles are internalized by clathrin-mediated endocytosis and are recycled for reuse, which thus completes the synaptic vesicle cycle. SV, synaptic vesicle CCV, clathrin-coated vesicle EE, early endosome. NOTE The use of arrows indicates a temporal sequence of events. Physical translocation of synaptic vesicles is unlikely to occur between the docking and fusion steps.

Manufacturer s Authorization Certificate issued by competent state authority. 

For transformation bacteria such as E. coli can uptake recombinant plasmid DNA by treating the cells with ice-cold CaCl2 until they reach a competent state in which they are ready to take up DNA. These cells are then presented with the recombinant plasmid and exposed briefly to a heat shock of 42°C which enables them to take up the DNA. 

In chap. 4 we argued that much can be learned by comparing the energetics of different competing states of a crystalline solid. One set of material parameters that can be understood on the basis of total energy methods like those presented in the previous chapter in conjunction with the methods of statistical mechanics are those related to the thermal response of materials. Indeed, one of the workhorses... 

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