Nuclear Structure and Nucleocytoplasmic Transport

Vocabulary flashcards covering key terms and concepts from the lecture notes on nuclear structure and nucleocytoplasmic transport.

Nucleus

The organelle that stores genetic information (DNA) and controls cellular function by regulating transcription and RNA processing.

Euchromatin

Less condensed chromatin that is generally transcriptionally active.

Heterochromatin

Highly condensed, gene-poor chromatin enriched in repressive histone marks; often located at the nuclear periphery.

Lamins

Intermediate filament proteins forming the nuclear lamina underneath the inner nuclear membrane; provide structural integrity and tether chromosomal regions.

Nuclear lamina

A meshwork of lamins that supports nuclear structure, organizes pores, and helps tether chromatin to the nuclear envelope.

Nucleolus

A membrane-less nuclear subcompartment where rRNA transcription and ribosome assembly occur; consists of three zones (FC, DFC, GC).

Nucleolar Organizing Region (NOR)

rDNA gene clusters within the nucleolus that drive rRNA transcription and nucleolus formation.

Fibrillar Center (FC)

Nucleolar zone where rDNA genes are located and transcription by RNA polymerase I begins

Dense Fibrillar Component (DFC)

Nucleolar zone where early processing of rRNA occurs; splicing and processing of rRNA

Granular Component (GC)

Nucleolar zone where assembly of pre-ribosomal subunits with ribosomal proteins takes place.

Speckles

Nuclear bodies rich in RNA processing components and storage; associated with RNA processing.

Cajal Bodies

Nuclear sites of RNA processing and maturation activities.

PML Bodies

Nuclear bodies with diverse functions; not fully understood (often enriched in regulatory proteins and RNAs).

Chromosome Territories (CTs)

Distinct, nonrandom spatial territories occupied by individual chromosomes within the nucleus.

Interphase nucleus organization

Concept that chromosomes and genes are organized nonrandomly in the nucleus, influencing gene expression; places telomeres near the nuclear periphery

Nuclear Pore Complex (NPC)

Gated channels in the nuclear envelope that mediate nucleocytoplasmic transport; built from many nucleoporins; central channel ~10 nm; ~5–10k pores per nucleus.

Nucleoporins (Nups)

FG-repeat proteins that assemble the NPC and create a selective barrier for transport.

FG repeats

Phenylalanine-Glycine repeats in nucleoporins that form the diffusion barrier within NPCs.

β-karyopherins (ß-karyopherins)

Transport receptors that recognize cargo with NLS/NES and interact with FG repeats to mediate NPC translocation.

Nuclear Localization Signal (NLS)

A short basic amino acid sequence that directs cargo into the nucleus; essential for efficient transport of cargo > ~40 kD.

Nuclear Export Signal (NES)

A signal sequence that directs cargo export from the nucleus via exportins in a RanGTP-dependent manner.

Ran GTPase

Small GTPase that cycles between RanGTP and RanGDP, establishing a nuclear-cytoplasmic gradient to direct transport.

RCC1 (RanGEF)

Ran Guanine nucleotide Exchange Factor; chromatin-associated; converts RanGDP to RanGTP in the nucleus.

RanGAP

Ran GTPase-activating protein; cytoplasmic; promotes GTP hydrolysis on Ran to RanGDP, helping maintain the gradient.

GAP/GEF/GDI (GTPase regulators)

Proteins regulating small GTPases: GAPs accelerate GTP hydrolysis (ON→OFF), GEFs promote GDP release and GTP binding (OFF→ON), and GDIs inhibit GDP dissociation.

Brownian ratchet

Model for NPC transport where karyopherins bind FG repeats and undergo cycles of binding/dissociation, enabling transit without direct energy input.

Telomeres

Specialized protective structures at chromosome ends; shorten with cell division; telomere length relates to aging; telomere replication is influenced by nuclear architecture.

Laminopathies

Diseases caused by LMNA (lamin A/C) mutations, leading to nuclear structural defects and tissue degeneration.

Progeria (Hutchinson-Gilford Progeria syndrome)

A dominant Lamin A/C mutation causing a toxic Lamin A that disrupts nuclear architecture and telomere replication, leading to premature aging.

Gene gating

Hypothesis that active genes relocate within the nucleus to functional compartments, often moving toward the interior to facilitate transcription.

Perinuclear heterochromatin

Heterochromatin frequently located near the nuclear envelope, contributing to gene silencing.

Transcription factor architecture

Transcription factors have a DNA-binding domain and a transcriptional activation domain; they recruit Mediator and RNA Pol II to promoters.

Nuclear mRNA processing and export (general concept)

Processed mRNA is segregated from transcription sites and exported through NPCs to the cytoplasm.