ion channel   Transmembrane protein complex that forms a water-filled channel across the lipid bilayer through which specific inorganic ions can diffuse down their electrochemical gradients. (Figure 11–22) ion-channel-coupled receptor (transmitter-gated ion channel, ionotropic receptor) Ion channel found at chemical synapses in the postsynaptic plasma membranes of nerve and muscle cells. Opens only in response to the binding of a specific extracellular neurotransmitter. The resulting inflow of ions leads to the generation of a local electrical signal in the postsynaptic cell. (Figures 15–6 and 11–35) ion-sensitive indicators Molecules whose light emission reflects the local concentration of a particular ion; some are luminescent (emitting light spontaneously) while others are fluorescent (emitting light on exposure to light).
IP3-gated Ca2+-release channel (IP3 receptor) Gated Ca2+ channel in the ER membrane that opens on binding cytosolic IP3, releasing stored Ca2+ into the cytosol. (Figure 15–29) iron–sulfur cluster   Electron-transporting group consisting of either two or four iron atoms bound to an equal number of sulfur atoms, found in a class of electron-transport proteins. (Figure 14–16)
J gene segment Short DNA sequences that encodes part of the variable region of light and heavy immunoglobulin chains and of α and β chains of T cell receptors. (Figures 24–28 and 24–29)
JAK–STAT signaling pathway Signaling pathway activated by cytokines and some hormones, providing a rapid route from the plasma membrane to the nucleus to alter gene transcription. Involves cytoplasmic Janus kinases (JAKs), and signal transducers and activators of transcription (STATs).
Janus kinases (JAKs) Cytoplasmic tyrosine kinases associated with cytokine receptors, which phosphorylate and activate transcription regulators called STATs.
junctional diversification The random loss and gain of nucleotides at joining sites during V(D)J recombination that occurs during B and T cell development when the cells are assembling the gene segments that encode their antigen receptors. It enormously increases the diversity of V-region coding sequences.
K+ leak channel K+-transporting ion channel in the plasma membrane of animal cells that remains open even in a “resting” cell.
karyotype   Display of the full set of chromosomes of a cell, arranged with respect to size, shape, and number.
keratin   Type of intermediate filament, commonly produced by epithelial cells.
kinase cascade Intracellular signaling pathway in which one protein kinase, activated by phosphorylation, phosphorylates the next protein kinase in the sequence, and so on, relaying the signal onward.
kinesin Member of one of the two main classes of motor proteins that use the energy of ATP hydrolysis to move along microtubules. (Figure 16–56) kinesin-1   Motor protein associated with microtubules that transports cargo within the cell; also called “conventional kinesin.” kinetic proofreading   A principle for increasing the specificity of catalysis. In the synthesis of DNA, RNA, and proteins, it refers to a time delay that begins with an irreversible step (such as ATP or GTP hydrolysis) and during which incorrect base pairs are more likely to dissociate than correct pairs.
kinetochore   Large protein complex that connects the centromere of a chromosome to microtubules of the mitotic spindle. (FIgure 17–30) kinetochore microtubule In the mitotic or meiotic spindle, a microtubule that connects the spindle pole to the kinetochore of a chromosome.