Loading Controls

Western blotting is a widely used technique for detecting and quantifying specific proteins in biological samples such as tissue lysates or cell extracts. To ensure reliable results, loading controls are commonly used as internal references in Western blot experiments.

Loading control antibodies detect proteins that are expected to be expressed at stable and relatively constant levels across different samples. These proteins, often referred to as housekeeping proteins, help verify that equal amounts of total protein have been loaded into each lane and that proteins have been transferred efficiently to the membrane during the Western blotting process.

By normalizing the signal of the protein of interest to a suitable Western blot loading control, researchers can correct for differences in sample loading, transfer efficiency, or experimental variation. Ideally, loading control proteins show high and ubiquitous expression and maintain consistent levels across different tissues, cell types, and experimental conditions.

Subcellular Localization of Common Loading Control Proteins

Loading Controls - Normalizing Protein Levels in Western Blots

Beta-Actin

Suitability: Whole cell lysates and cytoplasmic extracts

Molecular weight: ~42 kDa

β-Actin is one of the most commonly used loading controls in Western blot experiments. As a major component of the cytoskeleton, β-actin is expressed in most eukaryotic cell types and typically shows stable and abundant expression. For this reason, β-actin antibodies are widely used as Western blot loading controls to verify equal protein loading and transfer across lanes.

Actins belong to a highly conserved protein family consisting of three main isoform groups: α-actin, β-actin and γ-actin. While α-actin isoforms are mainly expressed in muscle tissues and form part of the contractile apparatus, β- and γ-actin are present in most cell types where they regulate cytoskeletal organization, cell motility and intracellular trafficking.

Note: Although β-actin is widely used as a housekeeping protein, its expression can vary under certain experimental conditions. Changes in cell growth conditions, cytoskeletal remodeling or interactions with extracellular matrix components may affect actin protein synthesis and should therefore be considered when selecting a Western blot loading control.

beta-Actin Antibodies

Glyceraldehyde 3-phosphate dehydrogenase (GAPDH)

Suitability: Whole cell lysates and cytoplasmic extracts

Molecular weight: ~37 kDa

Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) is one of the most frequently used loading controls in Western blot experiments. Because GAPDH is highly expressed in many cell types and tissues, GAPDH antibodies are commonly used as Western blot loading controls to verify equal protein loading and efficient transfer across samples.

GAPDH is a key metabolic enzyme that catalyzes the sixth step of glycolysis, contributing to the breakdown of glucose for energy production. In addition to its metabolic role, GAPDH has been implicated in a variety of cellular processes including transcriptional regulation, RNA binding and transport, DNA replication and repair, apoptosis, and vesicle trafficking between the endoplasmic reticulum and the Golgi apparatus.

Note: Although GAPDH is widely used as a housekeeping protein, its expression can vary under certain physiological and pathological conditions. Hypoxia, diabetes and some types of cancer have been reported to increase GAPDH expression in specific cell types, which should be considered when selecting an appropriate Western blot loading control.

GADPH antibodies

Histone H2B

Suitability: Nuclear extracts

Molecular weight: ~14 kDa

Histone H2B is commonly used as a nuclear loading control in Western blot experiments. Because histones are highly abundant proteins that are tightly associated with chromatin, Histone H2B antibodies are frequently used to confirm equal protein loading in nuclear extracts and to verify successful nuclear fractionation.

Histone H2B is one of the five major histone proteins (H1, H2A, H2B, H3 and H4) that form the structural core of chromatin in eukaryotic cells. Together with H2A, H3 and H4, Histone H2B contributes to the formation of nucleosomes, the fundamental units of chromatin often described as a "beads-on-a-string" structure. Each histone protein contains a globular core domain and flexible N-terminal tails that are subject to multiple post-translational modifications involved in chromatin regulation.

Note: Histone levels increase during DNA replication and prior to cell division. As a result, Histone H2B may not be suitable for comparing samples from different stages of the cell cycle, such as S-phase versus pre-S-phase cells.

Histone H2B antibodies

Proliferating Cell Nuclear Antigen (PCNA)

Suitability: Nuclear extracts

Molecular weight: ~36 kDa

Proliferating Cell Nuclear Antigen (PCNA) is frequently used as a nuclear loading control in Western blot experiments, particularly when analyzing proteins from nuclear extracts. Due to its nuclear localization and relatively abundant expression in proliferating cells, PCNA antibodies can be used to verify equal loading and efficient transfer of nuclear proteins.

PCNA is a highly conserved protein that functions as a cofactor of DNA polymerase δ in eukaryotic cells. During DNA replication, PCNA acts as a sliding DNA clamp that encircles the DNA duplex and increases the processivity of leading strand synthesis. By forming a ring-shaped complex around DNA, PCNA provides a stable platform for DNA polymerases and other proteins involved in replication and DNA repair.

Note: PCNA levels are closely linked to DNA replication and cell proliferation. In addition, PCNA can be rapidly degraded or modified when DNA damage response pathways are activated. Therefore, PCNA may not be suitable as a loading control in experiments involving DNA damage, cell cycle regulation or replication stress.

PCNA antibodies

TATA binding protein (TBP)

Suitability: Nuclear extracts

Molecular weight: ~38 kDa

TATA binding protein (TBP) is commonly used as a nuclear loading control in Western blot experiments. Because TBP is a constitutively expressed transcription factor localized in the nucleus, TBP antibodies are frequently used to confirm equal protein loading and transfer in nuclear extracts.

TBP is a core component of the transcription factor complex TFIID, which plays a central role in the initiation of gene transcription by RNA polymerase II. The protein binds specifically to the TATA box, a conserved DNA sequence located in the promoter region of many archaeal and eukaryotic genes. Through its interaction with other transcription factors, TBP helps assemble the RNA polymerase II pre-initiation complex (PIC), which is required for transcription initiation.

Note: TBP is most suitable as a loading control when analyzing nuclear extracts. It may not be appropriate for experiments in which DNA or chromatin-associated nuclear components have been removed during sample preparation.

TBP antibodies

Tubulin

Suitability: Whole cell lysates and cytoplasmic extracts

Molecular weight: ~50–55 kDa

Tubulin is commonly used as a loading control in Western blot experiments analyzing whole cell lysates or cytoplasmic extracts. Because tubulin proteins are highly conserved and abundantly expressed in most eukaryotic cells, α-tubulin and β-tubulin antibodies are frequently used as Western blot loading controls to verify equal protein loading and transfer across samples.

Tubulins are the major structural components of microtubules, which are formed by heterodimers of α- and β-tubulin. Microtubules are essential elements of the cytoskeleton and contribute to the maintenance of cell shape, intracellular transport of vesicles and organelles, and the movement of cilia and flagella. In addition, microtubules play a critical role during cell division by forming the mitotic spindle required for chromosome segregation during mitosis and meiosis.

Note: Tubulin expression levels can vary under certain experimental conditions, particularly in response to antimicrobial or antimitotic drugs that target microtubule dynamics. Such treatments may influence tubulin abundance and should therefore be considered when selecting an appropriate Western blot loading control.

Tubulin antibodies

Voltage-dependent anion-selective channel protein 1 (VDCA1/Porin)

Suitability: Mitochondrial extracts

Molecular weight: ~31 kDa

Voltage-dependent anion channel 1 (VDAC1), also known as mitochondrial porin, is widely used as a mitochondrial loading control in Western blot experiments. Because VDAC1 is a highly abundant protein located in the outer mitochondrial membrane, VDAC1 antibodies are commonly used to verify equal loading of mitochondrial proteins and to confirm mitochondrial enrichment in fractionation experiments.

VDAC1 is a conserved β-barrel membrane protein that forms a channel in the outer mitochondrial membrane. This channel allows the diffusion of ions and small metabolites between the mitochondria and the cytosol and plays an important role in mitochondrial metabolism and energy regulation. Due to its stable expression and mitochondrial localization, VDAC1 is frequently used as a marker protein for mitochondrial fractions in Western blot analysis.

Note: VDAC1 is most suitable as a loading control for mitochondrial extracts or mitochondrial fractionation experiments and should not be used as a universal loading control for whole cell lysates.

VDCA1/Porin antibodies