The contribution of RCK domains to human BK channel allosteric activation.

Large conductance voltage- and Ca(2+)-activated K(+) (BK) channels are potent regulators of cellular processes including neuronal firing, synaptic transmission, cochlear hair cell tuning, insulin release, and smooth muscle tone. Their unique activation pathway relies on structurally distinct regulatory domains including one transmembrane voltage-sensing domain (VSD) and two intracellular high affinity Ca(2+)-sensing sites per subunit (located in the RCK1 and RCK2 domains). Four pairs of RCK1 and RCK2 domains form a Ca(2+)-sensing apparatus known as the "gating ring.

Metal-driven operation of the human large-conductance voltage- and Ca2+-dependent potassium channel (BK) gating ring apparatus.

Large-conductance voltage- and Ca(2+)-dependent K(+) (BK, also known as MaxiK) channels are homo-tetrameric proteins with a broad expression pattern that potently regulate cellular excitability and Ca(2+) homeostasis. Their activation results from the complex synergy between the transmembrane voltage sensors and a large (>300 kDa) C-terminal, cytoplasmic complex (the "gating ring"), which confers sensitivity to intracellular Ca(2+) and other ligands. However, the molecular and biophysical operation of the gating ring remains unclear.

The RCK1 domain of the human BKCa channel transduces Ca2+ binding into structural rearrangements.

Large-conductance voltage- and Ca(2+)-activated K(+) (BK(Ca)) channels play a fundamental role in cellular function by integrating information from their voltage and Ca(2+) sensors to control membrane potential and Ca(2+) homeostasis. The molecular mechanism of Ca(2+)-dependent regulation of BK(Ca) channels is unknown, but likely relies on the operation of two cytosolic domains, regulator of K(+) conductance (RCK)1 and RCK2. Using solution-based investigations, we demonstrate that the purified BK(Ca) RCK1 domain adopts an alpha/beta fold, binds Ca(2+), and assembles into an octameric superstructure similar to prokaryotic RCK domains.

Tear lipocalin is the major endonuclease in tears.

Purpose: Human endonucleases are integral to apoptosis in which unwanted or potentially harmful cells are eliminated. The rapid turnover of ocular surface epithelium and microbial colonization of the eyelids are continual sources of DNA in tears. Here, we determine the principal sources of endonuclease activity in tears.

The RCK2 domain of the human BKCa channel is a calcium sensor.

Large conductance voltage and Ca(2+)-dependent K(+) channels (BK(Ca)) are activated by both membrane depolarization and intracellular Ca(2+). Recent studies on bacterial channels have proposed that a Ca(2+)-induced conformational change within specialized regulators of K(+) conductance (RCK) domains is responsible for channel gating. Each pore-forming alpha subunit of the homotetrameric BK(Ca) channel is expected to contain two intracellular RCK domains.

Tear lipocalin: evidence for a scavenging function to remove lipids from the human corneal surface.

Purpose: Lipid contamination of the cornea may create an unwettable surface and result in desiccation of the corneal epithelium. Tear lipocalin (TL), also known as lipocalin-1, is the principal lipid-binding protein in tears. TL has been shown to scavenge lipids from hydrophobic surfaces.

Interstrand loops CD and EF act as pH-dependent gates to regulate fatty acid ligand binding in tear lipocalin.

Tear lipocalin (TL), a major component of human tears, shows pH-dependent endogenous ligand binding. The structural and conformational changes associated with ligand release in the pH range of 7.5-3.

Tear lipocalin: potential for selective delivery of rifampin.

The potential of ligand binding proteins as drug carriers and delivery systems has recently sparked great interest. We investigated the potential of tear lipocalin (TL) to bind the antibiotic, rifampin, and the environmental conditions for controlled release. To determine if TL binds rifampin, gel filtration was used to isolate protein fractions of tears.

Resolving near-ultraviolet circular dichroism spectra of single trp mutants in tear lipocalin.

Near-ultraviolet circular dichroism (near-UV CD) spectra of tryptophan residues in proteins are complicated because the line shapes are derived from the overlap of both the 1L(a) and the 1L(b) electronic bands that vary independently. Contributing to this complexity, tryptophan near-UV CD spectra differ in the relative amplitude of the 0-0 vibronic band compared to the rest of the 1L(b) spectrum, an inherent feature that may result in poor fitting. To resolve this problem, a computer program that incorporated the separation of the 0-0 transition of 1L(b) component from the rest of the 1L(b) was written in LabVIEW and its amplitude was allowed to vary independently.

Relaxation of beta-structure in tear lipocalin and enhancement of retinoid binding.

Purpose: To study binding of retinoids to human tear lipocalin (TL) to assess factors influencing ligand affinity and delivery. Mechanistic features of retinoid interactions with TL were investigated, including the influence of the retinoid functional group on ligand affinity, the relative affinity of retinol versus fatty acids, the influence of relaxation of secondary structure in TL on ligand binding, the role of specific conserved hydrophobic residues in maintaining the rigidity of the secondary structure, and the potential release of retinol in a low-pH environment that promotes structural relaxation at lipid interfaces.

Methods: The binding and displacement of retinoids were monitored by quenching of protein fluorescence.

RET and anisotropy measurements establish the proximity of the conserved Trp17 to Ile98 and Phe99 of tear lipocalin.

Previous studies suggest that the conserved Trp17 on strand A of TL has a role in lipocalin stability and interacts, directly or indirectly, with Ile98 and Phe99 on strand G to influence ligand binding. Here, we determined the proximity of Trp17 to Ile98 and Phe99. Time-resolved fluorescence experiments showed resonance energy transfer between tryptophans at positions 17 and 98.