Cell Biology Terms Starting With K

Karyokinesis is the division of a cell's nucleus during mitosis or meiosis, in which duplicated chromosomes separate and migrate to opposite poles of the cell to produce two genetically complete nuclei.

During karyokinesis, duplicated chromosomes, each consisting of two sister chromatids joined at the centromere, attach to spindle microtubules at protein structures called kinetochores. Shortening of kinetochore microtubules during anaphase pulls sister chromatids toward opposite poles at a rate of roughly 1 micrometer per minute in mammalian cells. The process encompasses prophase, prometaphase, metaphase, anaphase, and telophase, and concludes when a nuclear envelope reassembles around each chromosome set.

Cytokinesis, the physical separation of the cytoplasm, typically follows but is mechanically independent; in some organisms, such as the early Drosophila embryo, karyokinesis repeats 13 times without any intervening cytokinesis, producing a syncytium of roughly 6,000 nuclei sharing a common cytoplasm.

Keap1 is a cytoplasmic adaptor protein that binds the transcription factor Nrf2 and targets it for ubiquitin-mediated proteasomal degradation under normal, unstressed cellular conditions.

Under basal conditions, Keap1 forms a homodimer that captures Nrf2 through two binding motifs on the transcription factor, presenting it to a Cullin-3 ubiquitin ligase complex that marks Nrf2 for degradation every 10 to 20 minutes. The protein contains 27 cysteine residues distributed across its domains, and these cysteines detect electrophiles and reactive oxygen species with differing sensitivities. Oxidative modification of critical cysteines, particularly C151, C273, and C288, causes conformational changes that prevent Nrf2 ubiquitination without releasing the transcription factor from the complex.

Newly synthesized Nrf2 then escapes degradation, accumulates in the cytoplasm, translocates to the nucleus, and activates more than 250 cytoprotective genes encoding antioxidant and detoxification enzymes. Keap1 loss-of-function mutations occur in approximately 15 percent of lung adenocarcinomas and 10 percent of papillary renal cell carcinomas, constitutively activating Nrf2 and conferring resistance to chemotherapy.

Kinesin is a motor protein that uses energy from ATP hydrolysis to transport cargo such as organelles and vesicles along microtubules toward the plus end in eukaryotic cells.

Kinesin motors contain two heavy-chain heads that bind to microtubules and hydrolyze ATP to generate a power stroke, stepping toward the microtubule plus end at roughly 800 nanometers per second. Each ATP hydrolysis event drives one approximately 8-nanometer step, so kinesin moves continuously along the microtubule track while tethered to its cargo through a tail domain. Different kinesin families transport diverse cargoes including synaptic vesicles, mitochondria, and mRNA particles.

In neurons, this directed transport is indispensable because diffusion alone cannot reliably deliver materials across axons that may exceed one meter in length in large mammals.

KRAS is a small GTPase protein that cycles between an active GTP-bound state and an inactive GDP-bound state to regulate cell proliferation, differentiation, and survival through downstream signaling pathways.

KRAS belongs to the RAS family of proto-oncogenes and operates as a molecular switch in receptor tyrosine kinase signaling cascades. When growth factors bind cell surface receptors, guanine nucleotide exchange factors stimulate KRAS to exchange GDP for GTP, activating downstream effectors including the RAF-MEK-ERK and PI3K-AKT pathways. Intrinsic GTPase activity normally returns KRAS to its inactive state within minutes, but point mutations at codons 12, 13, or 61 impair this hydrolysis and lock the protein in its active conformation.

Approximately 25 percent of all human cancers carry KRAS mutations, with pancreatic ductal adenocarcinoma showing mutation rates exceeding 90 percent. The G12C substitution, present in roughly 13 percent of non-small cell lung cancers, became the first directly druggable KRAS variant when the FDA approved sotorasib in May 2021.