Alternative splice isoforms of small conductance calcium-activated SK2 channels differ in molecular interactions and surface levels

Small conductance Ca²⁺-sensitive potassium (SK2) channels are voltage-independent, Ca²⁺-activated ion channels that conduct potassium cations and thereby modulate the intrinsic excitability and synaptic transmission of neurons and sensory hair cells. In the cochlea, SK2 channels are functionally coupled to the highly Ca²⁺ permeant α9/10-nicotinic acetylcholine receptors (nAChRs) at olivocochlear postsynaptic sites. SK2 activation leads to outer hair cell hyperpolarization and frequency-selective suppression of afferent sound transmission. These inhibitory responses are essential for normal regulation of sound sensitivity, frequency selectivity, and suppression of background noise. However, little is known about the molecular interactions of these key functional channels. Here we show that SK2 channels co-precipitate with α9/10-nAChRs and with the actin-binding protein α-actinin-1. SK2 alternative splicing, resulting in a 3 amino acid insertion in the intracellular 3′ terminus, modulates these interactions. Further, relative abundance of the SK2 splice variants changes during developmental stages of synapse maturation in both the avian cochlea and the mammalian forebrain. Using heterologous cell expression to separately study the 2 distinct isoforms, we show that the variants differ in protein interactions and surface expression levels, and that Ca²⁺ and Ca²⁺-bound calmodulin differentially regulate their protein interactions. Our findings suggest that the SK2 isoforms may be distinctly modulated by activity-induced Ca²⁺ influx. Alternative splicing of SK2 may serve as a novel mechanism to differentially regulate the maturation and function of olivocochlear and neuronal synapses.     

Hemoglobin Dallas (alpha 97(G4)Asn-->Lys): functional characterization of a high oxygen affinity natural mutant.

Hemoglobin Dallas, an alpha-chain variant with a substitution of lysine for asparagine at position 97(G4), was found to have increased oxygen affinity (p1/2 = 1 mmHg at pH 7.3 and 20 degrees C), diminished cooperativity (n, the Hill coefficient = 1.7) and reduced Bohr effect (about 50%). Addition of allosteric effectors (such as 2,3-diphosphoglycerate, inositol hexakisphosphate and bezafibrate) led to a decrease in oxygen affinity and increase in cooperative energy. Kinetic studies at pH 7.0 and 20 degrees C revealed that (i), the overall rate of oxygen dissociation is 1.4-fold slower than that for HbA and (ii), the carbon monoxide dissociation rate is unaffected. The abnormal properties of this hemoglobin variant can be attributed to a more 'relaxed' T-state.     

Circadian clock adjustment to plant iron status depends on chloroplast and phytochrome function

Plant chloroplasts are not only the main cellular location for storage of elemental iron (Fe), but also the main site for Fe, which is incorporated into chlorophyll, haem and the photosynthetic machinery. How plants measure internal Fe levels is unknown. We describe here a new Fe‐dependent response, a change in the period of the circadian clock. In Arabidopsis, the period lengthens when Fe becomes limiting, and gradually shortens as external Fe levels increase. Etiolated seedlings or light‐grown plants treated with plastid translation inhibitors do not respond to changes in Fe supply, pointing to developed chloroplasts as central hubs for circadian Fe sensing. Phytochrome‐deficient mutants maintain a short period even under Fe deficiency, stressing the role of early light signalling in coupling the clock to Fe responses. Further mutant and pharmacological analyses suggest that known players in plastid‐to‐nucleus signalling do not directly participate in Fe sensing. We propose that the sensor governing circadian Fe responses defines a new retrograde pathway that involves a plastid‐encoded protein that depends on phytochromes and the functional state of chloroplasts.     

Cytochrome-c oxidase. Subunit structure and proton pumping

This article reviews the significance of the subunit structure of cytochrome-c oxidase in proton pumping and in particular summarizes available evidences for or against a role of subunit III in the control of this important function of the enzyme.     

Is there a Root effect in Xenopus hemoglobin?

The reaction of Xenopus hemoglobin with oxygen and carbon monoxide has been reinvestigated over the pH range 8.5-6.0, in the absence and presence of organic phosphates (2,3-diphosphoglycerate or inositol hexakisphosphate), to establish if the tetramer can be stabilized in a T-quaternary state by protons and polyphosphate; the equilibrium and kinetic data indicate that Xenopus hemoglobin does exhibit a Root effect. These new results are discussed with reference to those reported by Bridges et al. [(1985) Resp. Physiol. 61, 125-136] on Xenopus blood and, more generally, to the molecular definition and the structural basis of the Root effect as an extreme form of the Bohr effect.