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A protein subunit is an individual polypeptide component of a protein complex. The number of protein subunits counts chain copies, whereas the number of subunit types counts distinct kinds of chain. An assembly $A_2{B}_2$ has four protein subunits of two types. A protein domain is a region within a chain and need not be a separate protein subunit; the arrangement of several chains belongs to protein quaternary structure.

The alpha/beta tubulin heterodimer illustrates the distinction: it contains two related but different polypeptides, not two domains cut out of one chain. The 1998 electron-crystallographic model resolved the alpha/beta architecture at $3.7$ angstrom resolution, using zinc-induced tubulin sheets.^[ref]

Subunit counting can also become an optical experiment. In total internal reflection fluorescence microscopy, tagged membrane proteins can be observed as isolated fluorescent spots; successive photobleaching steps reveal individual fluorescent tags disappearing. Subunit counting in membrane-bound proteins by Maximilian H. Ulbrich and Ehud Y. Isacoff used this approach on membrane proteins expressed in frog oocytes. For NR1/NR3B receptor assemblies, tagging one component type mainly gave two steps, while tagging both gave four, supporting a two-plus-two arrangement.^[ref]

A count of visible steps need not equal the true number of protein subunits: some tags fail to fluoresce, and two tags may bleach too close together to resolve. The experiment therefore combines step counts with a model of labeling and detection, rather than interpreting every two-step trace as proof of a two-chain protein complex.^[ref]

A proteoglycan consists of a core protein bearing one or more covalently attached glycosaminoglycan chains. These chains are polysaccharides containing amino sugars. Thus the molecule combines a sequence-defined polypeptide with long carbohydrate components, whose number and composition can vary.^[ref]

An illuminating example comes from cartilage, where individual proteoglycans can associate with hyaluronic acid into larger aggregates. A proteoglycan molecule and an aggregate of many such molecules are different organizational levels: a change in aggregate size need not mean that every core protein has changed length.

In 1974, researchers digested bovine cartilage aggregates with enzymes and separated the resulting components by chromatography. They isolated a core-protein-derived fraction and a smaller linking-protein fraction that both retained binding to hyaluronic acid. Disassembly and selective recovery of binding components helped distinguish the carbohydrate-rich molecule from the interactions holding a larger aggregate together.^[ref]

A release factor is a protein involved in terminating biological translation and releasing the completed polypeptide at a stop codon.^[ref]

The ribosomal exit tunnel is the passage through the large ribosomal subunit leading from the peptidyl transferase center toward the exterior of the ribosome. The growing polypeptide enters this passage during biological translation. Its walls contain ribosomal RNA together with contributions from ribosomal proteins; the tunnel is part of the molecular machine, rather than an opening through the cell membrane.^[ref]

The tunnel can also help distinguish particular nascent sequences. A 2002 experiment identified a SecM peptide motif that arrests elongation. Mutations in 23S ribosomal RNA or ribosomal protein L22 could bypass this arrest; the affected residues faced the tunnel's constriction. Changing tunnel components changed the response to the nascent product, supporting a functional interaction between the polypeptide and its surroundings.^[ref]

Consider two chains containing the same number of amino acid residues in different orders. Equal length alone would not imply identical passage or stalling behaviour, because their side chains encounter different local environments. The SecM result makes that distinction experimentally meaningful: the ribosomal exit tunnel can participate in sequence-dependent regulation, while remaining the route followed by many unrelated polypeptides.

A ribosome is the molecular machinery for biological translation: it reads messenger RNA and joins amino acids into a polypeptide that can become a protein. Both prokaryotes and eukaryotes have ribosomes. A ribosome consists of ribosomal RNA and proteins organized into small and large subunits, with no enclosing membrane. Ribosomal RNA contributes to both structure and the active site that forms peptide bonds; a ribosome is therefore not merely a collection of proteins. See Clark, Douglas and Choi, Biology 2e, 4.3: ribosome composition and Nissen et al., The structural basis of ribosome activity in peptide bond synthesis, Science 289, 920–930 (2000).

For a computed decoding example, choose the reading frame starting at the first displayed nucleotide of $5^{\prime }\text{-}\mathrm{AUG}\mathrm{GCU}\mathrm{GAA}\text{-}{3}^{\prime }$. Under the standard genetic code, AUG specifies methionine (Met), GCU specifies alanine (Ala), and GAA specifies a glutamic acid residue (Glu) in the growing polypeptide. Nine nucleotides divide into three codons, giving the ordered three-amino acid residue segment Met–Ala–Glu. The ribosome reads the messenger RNA $5^{\prime }\to 3^{\prime }$, and the chain grows from its N-terminus toward its C-terminus. If synthesis begins at this AUG in an appropriate eukaryotic initiation context, these are the first three amino acid residues before any subsequent processing. See Clark, Douglas and Choi, Biology 2e, 15.1: standard codon assignments.

For this three-amino acid residue segment, joining the three delivered amino acids requires two peptide bonds, one between Met and Ala and another between Ala and Glu. There is no stop codon in the displayed nucleotide sequence, so this is a growing segment rather than a complete released protein. An isolated nine-nucleotide RNA is not claimed to supply all signals needed for initiation. This small calculation separates three jobs: the messenger RNA specifies order, transfer RNA delivers the amino acids, and the ribosome catalyzes their joining. Each new amino acid residue is added at the C-terminus. See Clark, Douglas and Choi, Biology 2e, 15.5: initiation and chain elongation.

The beauty of a ribosome is its reusable chemistry: the same machinery can assemble many different proteins by following different messenger RNA templates. The genetic code connects an alphabet of nucleotides to an alphabet of amino acids, while transfer RNA links recognition of the message to delivery of building material. Changing the message can therefore change the product without building a different ribosome for every protein.

In 1955 George E. Palade described small particles in the cytoplasm, both free and associated with membranes, in A small particulate component of the cytoplasm, Journal of Biophysical and Biochemical Cytology 1, 59–68. The paper documents the structural observations, rather than the full modern decoding mechanism. Complementary biochemical work by Hoagland, Stephenson, Scott, Hecht and Zamecnik described a soluble RNA intermediate in protein synthesis in 1958, helping establish the role now assigned to transfer RNA; see A soluble ribonucleic acid intermediate in protein synthesis, Journal of Biological Chemistry 231, 241–257. Together, structural observations and biochemical experiments connected visible cellular machinery with its working parts.

Processes recognizing problematic stalled ribosomes and managing their incomplete polypeptides and associated components.

A serine unit within a polypeptide. Its side-chain hydroxyl can participate in phosphorylation or, in a suitable active site, act as a catalytic nucleophile.

In the standard genetic code, UAA, UAG and UGA are stop codons: they signal termination of biological translation rather than an amino acid assignment. See Clark, Douglas and Choi: stop codons.

A stop codon supplies no additional amino acid residue. In the constructed AUG GCU UUC UGA example, replacing UGA by UAA preserves termination after the same three amino acid residues. Replacing UUC by UAA instead terminates after methionine and alanine. These are consequences of the stated genetic code, not measurements of expression efficiency.

At a standard stop codon, a release factor promotes release of the polypeptide from the ribosome. Ordinary delivery of an amino acid by transfer RNA does not supply a “stop amino acid”.^[ref]

A suppressor mutation is a second genetic change that reduces or counteracts a specified effect of another change. The resulting phenotype can move toward the original functional state while the original altered DNA sequence remains present. Suppression can act through another position in the same gene or through a different gene; recovery of one readout need not restore every property of the organism.

A constructed reading frame example makes the distinction visible. A one-nucleotide insertion changes the downstream grouping of codons. A second, later one-nucleotide deletion can restore the original grouping after the second change, because the net length change is $1-1=0$. The sequence between the two changes can still specify different amino acids. Two additional inserted nucleotides would likewise give a net change of three, restoring phase downstream without recreating the original nucleotide sequence. Whether function recovers remains an experimental question.

Suppressor screens also identify mutations that bypass a defined regulatory phenotype. In a 2002 SecM study, mutations in 23S ribosomal RNA and ribosomal protein L22 relieved ribosome stalling. The affected positions faced the ribosomal exit tunnel constriction, linking the altered response to the environment around the nascent polypeptide.^[ref] Such a screen finds a functional relationship; by itself it does not prove that every suppressor physically contacts the original altered component.

A tryptophan residue is tryptophan incorporated in a polypeptide. Its aromatic group contributes to ultraviolet absorption and intrinsic protein fluorescence, with optical behaviour dependent on the surrounding structure.

V. Sasisekharan helped map geometrically allowed polypeptide backbone configurations. Representing the backbone by torsion angles exposes combinations excluded by steric overlap, connecting molecular geometry with restrictions on protein structure.^[ref]