Cell Biology Terms Starting With R

Receptor is a protein that recognizes a specific signal molecule or stimulus and, upon binding that signal, initiates a response within the cell or organism.

G-protein-coupled receptors bind extracellular signals like epinephrine and activate intracellular G proteins that modulate second messenger production and ion channel activity. Ion channel receptors open or close directly in response to ligand binding, producing rapid electrical responses in neurons and muscle cells within milliseconds. Nuclear receptors, such as steroid hormone receptors, reside inside the cell and translocate to the nucleus after ligand binding, where they regulate gene transcription by binding specific DNA sequences.

Precise three-dimensional complementarity between the signal molecule and the receptor’s binding pocket determines the specificity of each receptor-ligand interaction.

Receptor Tyrosine Kinase is a class of cell-surface receptor that, upon binding a ligand, phosphorylates tyrosine residues on itself and on intracellular target proteins to initiate downstream signaling cascades.

Upon ligand binding, receptor tyrosine kinases undergo trans-autophosphorylation, meaning each receptor in a pair phosphorylates tyrosine residues on its partner, creating docking sites for proteins containing SH2 or PTB domains. Growth factor receptors such as EGFR and PDGFR then initiate phosphorylation cascades including the MAPK and PI3K pathways, controlling cell proliferation, differentiation, and survival. Mutation or overexpression of receptor tyrosine kinases drives numerous cancers; HER2 amplification occurs in roughly 20 percent of breast cancers, making these receptors major drug targets for therapies like trastuzumab.

The kinase domain catalyzes the transfer of a phosphate group from ATP to target tyrosine residues, permanently altering the activity or protein-protein interactions of the substrate.

RNA processing is the set of co-transcriptional and post-transcriptional modifications that convert a newly synthesized RNA molecule into a mature, functional form ready for use by the cell.

A 7-methylguanosine cap added to the 5′ end of pre-mRNA while transcription is still in progress protects the transcript from exonuclease degradation and recruits the ribosome during translation initiation. Splicing removes non-coding introns and joins coding exons through two transesterification reactions catalyzed by the spliceosome, a dynamic assembly of five small nuclear RNAs and more than 150 proteins that recognizes conserved GU and AG dinucleotides at splice sites. At the 3′ end, cleavage and polyadenylation factors add a poly-A tail of roughly 200 to 250 adenine nucleotides in mammals, increasing mRNA stability and translation efficiency.

Alternative splicing of a single pre-mRNA can generate structurally distinct proteins; the neurexin genes in humans produce thousands of isoforms through combinatorial exon inclusion, contributing to the molecular diversity of synaptic connections.

Rough ER is the ribosome-studded region of the endoplasmic reticulum where proteins destined for secretion, membrane insertion, or delivery to organelles are synthesized and begin folding.

Signal recognition particle recognizes signal sequences on nascent proteins during translation and directs ribosome complexes to dock at the rough ER membrane via the translocon protein complex. Proteins synthesized on rough ER ribosomes are co-translationally inserted into the ER lumen or embedded in the membrane, then transported via COPII-coated vesicles to the Golgi apparatus, lysosomes, or plasma membrane. Plasma cells producing antibodies concentrate rough ER to an extraordinary degree, synthesizing immunoglobulins at rates exceeding 2,000 molecules per second per cell.

Chaperone proteins within the rough ER lumen, including BiP, assist proper protein folding and retain misfolded proteins for degradation through the unfolded protein response.