Enhancing ubiquitin crystallization through surface-entropy reduction.

Ubiquitin has many attributes suitable for a crystallization chaperone, including high stability and ease of expression. However, ubiquitin contains a high surface density of lysine residues and the doctrine of surface-entropy reduction suggests that these lysines will resist participating in packing interactions and thereby impede crystallization. To assess the contributions of these residues to crystallization behavior, each of the seven lysines of ubiquitin was mutated to serine and the corresponding single-site mutant proteins were expressed and purified.

A role for the polarity complex and PI3 kinase in branch formation within retinotectal arbors of zebrafish.

The developing zebrafish retinotectal arbors make many trial branches with synapses but most are retracted. With NMDA blockers, branches are withdrawn at a higher rate, and a synapse on a branch not only stabilizes that branch, but biases new branches to form nearby. Here, we tested whether new branch formation requires the polarity complex, which is essential for organizing the cytoskeleton in initial axon formation.

GAP43 phosphorylation is critical for growth and branching of retinotectal arbors in zebrafish.

Visual activity acts via NMDA Receptors to refine developing retinotectal maps by shaping retinal arbors. Retinal axons add and delete transient branches, and the dynamic rates increase when MK801 blocks NMDARs, as if this prevents release of a stabilizing signal. Ca(++) entry through NMDARs activates phospholipase A2 (cPLA2) to release arachidonic acid (AA), which taps into a presynaptic growth control mechanism.

Fireflies at one hundred plus: a new look at flash control.

The mysterious process by which fireflies can control their flashing has inspired over a century of careful observation but has remained elusive. Many studies have implicated oxygen as the controlling element in the photochemical reaction, and the discovery of nitric oxide synthetase (NOS) in the lantern has suggested that nitric oxide (NO) may control oxygen access to the light-emitting photocytes, thereby triggering the flash. However, there are several drawbacks to oxygen as a controlling agent, and in view of the prominence of peroxisomes in lantern morphology and biochemistry, we suggest that it is hydrogen peroxide that triggers the flash, and we present a model by which this may take place.

Activity-driven sharpening of the retinotectal projection: the search for retrograde synaptic signaling pathways.

Patterned visual activity, acting via NMDA receptors, refines developing retinotectal maps by shaping individual retinal arbors. Because NMDA receptors are postsynaptic but the retinal arbors are presynaptic, there must be retrograde signals generated downstream of Ca(++) entry through NMDA receptors that direct the presynaptic retinal terminals to stabilize and grow or to withdraw. This review defines criteria for retrograde synaptic messengers, and then applies them to the leading candidates: nitric oxide (NO), brain-derived neurotrophic factor (BDNF), and arachidonic acid (AA).

Presynaptic protein kinase C controls maturation and branch dynamics of developing retinotectal arbors: possible role in activity-driven sharpening.

Visual activity refines developing retinotectal maps and shapes individual retinal arbors via an NMDA receptor-dependent mechanism. As retinal axons grow into tectum, they slow markedly and emit many transient side branches behind the tip, assuming a "bottlebrush" morphology. Some branches are stabilized and branch further, giving rise to a compact arbor.

Myosin light chain phosphorylation and growth cone motility.

According to the treadmill hypothesis, the rate of growth cone advance depends upon the difference between the rates of protrusion (powered by actin polymerization at the leading edge) and retrograde F-actin flow, powered by activated myosin. Myosin II, a strong candidate for powering the retrograde flow, is activated by myosin light chain (MLC) phosphorylation. Earlier results showing that pharmacological inhibition of myosin light chain kinase (MLCK) causes growth cone collapse with loss of F-actin-based structures are seemingly inconsistent with the treadmill hypothesis, which predicts faster growth cone advance.