A chiefly monomeric oxygen-binding globin. Its conserved core and haem pocket resemble those of haemoglobin subunits, while its assembly and physiological role differ.

An iron–sulfur electron-transfer protein. Photosynthetic ferredoxin receives electrons downstream of photosystem I and supplies reduction pathways including NADP reduction.

A cytotoxic-lymphocyte pore-forming protein supporting delivery of granzymes during target-cell killing.

Pore formation helps granzymes reach the target-cell interior after cytotoxic-cell contact. This makes membrane permeabilization and the subsequent death programme distinguishable stages; detection of one effector alone does not establish that target killing was completed. ^[ref]

A bacterial regulator whose expression can depend on EF-P-sensitive proline sequences.^[ref]

An adaptor protein connects other components of a molecular pathway through binding interactions. Multiple interaction domains can bring a receptor, an enzyme and a cytoskeletal partner into one local assembly. The adaptor need not itself catalyse the pathway’s chemical reaction. Its abundance and localization can still regulate signalling by changing which partners encounter one another. Nck illustrates an adaptor linking phosphorylation-dependent recognition to downstream assemblies. ^[ref]

An intrinsically disordered protein contains substantial regions that do not adopt one persistent folded structure under the specified conditions. Such regions can bind several partners, acquire structure on binding or support flexible multivalent assemblies. Disorder is therefore compatible with biological function. It is also condition-dependent: a disordered region can change its conformational distribution with binding, modification or environment rather than remaining an entirely structureless chain. ^[ref]

A DNA-binding protein associates physically with DNA. Some recognize selected base patterns, while others preferentially recognize DNA shape, ends or damage. Transcription factors regulate expression, histones organize packaging and repair factors inspect damaged substrates. These roles illustrate why binding alone does not establish a specific regulatory outcome. Sequence selectivity, affinity and cellular occupancy are distinct measurable properties. ^[ref]

Ligand binding is association of a ligand with a binding partner, such as a receptor or another protein. For a simple equilibrium $P+LightleftharpoonsPL$, the dissociation constant is $K_d=[P][L]/[PL]$. If free ligand is controlled and sites are independent, fractional occupancy is $[L]/(K_d+[L])$. Half-occupancy occurs at free $[L]=K_d$. This statement concerns binding; activation can require additional conformational or assembly steps. ^[ref]

Streptavidin is a biotin-binding protein widely used to capture or label biotinylated material. Its tetrameric assembly provides multiple binding sites and can therefore crosslink tagged components. Strong binding makes the pair useful for retention during measurements, but multivalency and steric accessibility affect the resulting geometry. Capture of a biotinylated molecule demonstrates access to the tag, not necessarily preservation of every native molecular interaction. ^[ref]

An epitope tag is a short recognizable sequence fused to a protein so that an antibody can detect or isolate it. The FLAG tag is one example. A shared tag permits the same detection reagent to follow different engineered products. The fusion can affect folding, localization or activity, so detection of the tag does not alone establish that the tagged protein retains normal function. ^[ref]

An iron-storage protein assembly. translational regulation through an iron-responsive element helps match storage capacity to iron availability; the iron-free protein is apoferritin.

A Drosophila sex-determination regulator whose productive expression depends on the splice pattern of its precursor RNA.

Protein degradation breaks proteins into smaller products and helps control protein abundance and quality. The ubiquitin-proteasome system selectively targets many intracellular proteins, while lysosomes degrade other material delivered by intracellular trafficking or autophagy. Synthesis and removal jointly determine abundance, so reduced protein level need not imply reduced transcription. Controlled degradation also makes some regulatory changes rapid and effectively irreversible for the individual protein molecule. ^[ref]

A fly centrosomal protein involved in centrosome organization and the centrosome orientation checkpoint.

A protein that binds and transports a lipid between membranes or promotes its exchange at a contact site.

A calcium-binding regulatory protein that can also act as a myosin-associated light chain in appropriate motor classes.

A protein carrying covalently attached growing acyl intermediates through fatty-acid synthesis.

A tetraheme inner-membrane-associated cytochrome in Shewanella that links quinol oxidation to periplasmic electron-transfer routes.

A surface-exposed decaheme cytochrome in the Shewanella extracellular electron-transfer system.

An actin-related bacterial cytoskeletal protein that helps organize and orient cell-wall growth in many rod-shaped bacteria.