Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

G:22 Glossary
NMDA receptor Subclass of glutamate-gated ion channel
in the mammalian central nervous system critical for longterm
potentiation and long-term depression. NMDA-receptor
channels are doubly gated, opening only when glutamate is
bound to the receptor and, simultaneously, the membrane is
strongly depolarized.
NO synthase (NOS) Enzyme that synthesizes nitric oxide
(NO) by the deamination of arginine. (Figure 15–40B)
NOD-like receptors (NLRs) Large family of pattern
recognition receptors (PRRs) with leucine-rich repeat motifs;
they are exclusively cytoplasmic and recognize a distinct set of
microbial molecules.
nonclassical cadherins Large family of cadherins that are
more distantly related in sequence than classical cadherins and
include proteins involved in adhesion (including protocadherins,
desmocollins, and desmogleins) and signaling.
noncoding RNA An RNA molecule that is the final product
of a gene and does not code for protein. These RNAs serve as
enzymatic, structural, and regulatory components for a wide
variety of processes in the cell.
nondisjunction Event occurring occasionally during meiosis
in which a pair of homologous chromosomes fails to separate
so that the resulting germ cell has either too many or too few
chromosomes.
nonenveloped virus Virus consisting of a nucleic acid core
and a protein capsid only. (Figure 23–18C,D)
nonhomologous end joining A DNA repair mechanism for
double-strand breaks in which the broken ends of DNA are
brought together and rejoined by DNA ligation, generally with
the loss of one or more nucleotides at the site of joining.
nonretroviral retrotransposons Type of transposable
element that moves by being first transcribed into an RNA copy
that is converted to DNA by reverse transcriptase then inserted
elsewhere in the genome. The mechanism of insertion differs
from that of the retroviral-like transposons. (Table 5–4, p. 288)
nonsense-mediated mRNA decay Mechanism for
degrading aberrant mRNAs containing in-frame internal stop
codons before they can be translated into protein. (Figure 6–76)
normal flora The human microbiota consisting of
approximately 10 14 bacterial, fungal, and protozoan cells,
representing thousands of microbial species.
Notch Transmembrane receptor protein (and latent
transcription regulator) involved in many cell-fate choices in
animal development, for example in the specification of nerve
cells from ectodermal epithelium. Its ligands are cell-surface
proteins such as Delta and Serrate. (Figure 15–59)
NSF Hexameric ATPase that disassembles a complex of a
v-SNARE and a t-SNARE. (Figure 13–20)
nuclear envelope Double membrane (two bilayers)
surrounding the nucleus. Consists of an outer and inner
membrane and is perforated by nuclear pores. The outer
membrane is continuous with the endoplasmic reticulum.
(Figures 4–9 and 12–7)
nuclear export receptors Bind to both the export signal and
nuclear pore complex proteins to guide their cargo through the
nuclear pore complex to the cytosol.
nuclear export signal Sorting signal contained in the
structure of molecules and complexes, such as nuclear RNPs
and new ribosomal subunits, that are transported from the
nucleus to the cytosol through nuclear pore complexes.
(Figure 12–13)
nuclear import receptors Recognize nuclear localization
signals to initiate nuclear import of proteins containing the
appropriate nuclear localization signal.
nuclear lamin Protein subunit of the intermediate filaments
that form the nuclear lamina.
nuclear lamina Fibrous meshwork of proteins on the inner
surface of the inner nuclear membrane. It is made up of a
network of intermediate filaments formed from nuclear lamins.
nuclear localization signal (NLS) Signal sequence or signal
patch found in proteins destined for the nucleus that enables
their selective transport into the nucleus from the cytosol
through the nuclear pore complexes. (Figures 12–9 and 12–13)
nuclear magnetic resonance (NMR) spectroscopy NMR
is the resonant absorption of electromagnetic radiation at a
specific frequency by atomic nuclei in a magnetic field, due to
flipping of the orientation of their magnetic dipole moments.
The NMR spectrum provides information about the chemical
environment of the nuclei. NMR is used widely to determine the
three-dimensional structure of small proteins and other small
molecules. The principles of NMR are also used for medical
diagnostic purposes in magnetic resonance imaging (MRI).
(Figure 8–22)
nuclear pore complex (NPC) Large multiprotein structure
forming an aqueous channel (the nuclear pore) through the
nuclear envelope that allows selected molecules to move
between nucleus and cytoplasm. (Figure 12–8)
nuclear receptor superfamily Intracellular receptors for
hydrophobic signal molecules such as steroid and thyroid
hormones and retinoic acid. The receptor-ligand complex acts
as a transcription factor in the nucleus. (Figure 15–65)
nuclear transport receptor (karyopherin) Protein that
escorts macromolecules either into or out of the nucleus:
nuclear import receptor or nuclear export receptor. (Figure
12–13)
nucleolus A prominent structure in the nucleus where rRNA is
transcribed and ribosomal subunits are assembled. (Figure 4–9)
nucleoporin Any of a number of different proteins that make
up nuclear pore complexes.
nucleosome Beadlike structure in eukaryotic chromatin,
composed of a short length of DNA wrapped around an
octameric core of histone proteins. The fundamental structural
unit of chromatin. (Figures 4–22 and 4–23)
nucleotide Nucleoside with one or more phosphate groups
joined in ester linkages to the sugar moiety. DNA and RNA are
polymers of nucleotides. (Panel 2–6, pp. 100–101)
nucleotide excision repair Type of DNA repair that corrects
damage of the DNA double helix, such as that caused by
chemicals or UV light, by cutting out the damaged region on
one strand and resynthesizing it using the undamaged strand
as template. Compare base excision repair. (Figure 5–41)
O-linked glycosylation Addition of one or more sugars to a
hydroxyl group on a protein.
obligate pathogens Bacteria that can only replicate inside
their host.
olfactory receptors G-protein-coupled receptors on the
modified cilia of olfactory receptor neurons that recognize
odors. The receptors activate adenylyl cyclase via an olfactory-