Recombinant Rat Protein orai-3 (Orai3)

What is Orai3 and how does it differ from other Orai proteins?

Orai3 is one of three mammalian Orai proteins (Orai1, Orai2, and Orai3) that function as essential pore-forming components of calcium channels. While the pore-forming transmembrane domains of all three Orai proteins show a high degree of conservation (approximately 82%), Orai3 exhibits significant structural differences in other regions . Compared to Orai1, major differences exist in:

• The cytosolic N-terminus: Only 34% sequence identity with Orai1; Orai3's N-terminus is shorter (approximately 65 amino acids compared to Orai1's 90 amino acids) and lacks the clusters of prolines and arginines seen in Orai1

• The cytosolic C-terminus: 46% sequence identity with Orai1

• The extracellular loop between transmembrane domains 3 and 4: Only 21% sequence identity; this loop is 72 amino acids long in Orai3 compared to only 38 amino acids in Orai1

These structural differences contribute to unique functional properties of Orai3, including its response to pharmacological agents and its role in forming distinct calcium channel complexes.

What is the physiological role of Orai3 in calcium signaling?

Orai3 plays multiple roles in calcium signaling depending on the cellular context:

• In store-operated calcium entry (SOCE): Both Orai3 and Orai1 mediate native Ca²⁺ release-activated Ca²⁺ (CRAC) channels in B cells

• In arachidonic acid-regulated calcium (ARC) channels: Orai3 is an essential component, forming heteromeric channels with Orai1 that are activated independently of store depletion

• In B lymphocyte function: The combined deletion of Orai1 and Orai3 (but not Orai3 alone) impairs SOCE, proliferation, survival, nuclear factor of activated T cells (NFAT) activation, and metabolic reprogramming in response to antigenic stimulation

Interestingly, Orai3 expression changes dynamically during B cell activation. Stimulation with anti-IgM+anti-CD40 or LPS significantly increases Orai3 expression, while stimulation with anti-IgM or anti-CD40 alone does not significantly affect Orai3 levels .

How can I effectively study Orai3 function in cellular models?

When investigating Orai3 function, consider the following methodological approaches:

Genetic manipulation techniques:

• siRNA-mediated knockdown: Useful for examining the contribution of Orai3 to calcium signaling; siRNA-mediated knockdown of either Orai1 or Orai3 markedly inhibits ARC channel currents

• Overexpression studies: When overexpressing Orai3, consider that it may have different effects depending on the expression levels of other proteins. In cells stably expressing STIM1, overexpression of Orai3 alone has no effect on calcium currents, but ARC channel currents are specifically increased by expression of Orai3 in cells stably expressing Orai1

• Dominant-negative approaches: Expression of dominant-negative mutant Orai3 can specifically reduce currents through ARC channels without affecting CRAC channels

Experimental considerations:

• Expression level control: The precise effects observed critically depend on which of the three proteins necessary for effective ARC channel activity (STIM1, Orai1, and Orai3) are rate-limiting under the specific conditions employed

• Multiple protein expression: If both Orai1 and Orai3 are important for the channel function under study, experiments may require the transient overexpression of multiple components

What are the pharmacological tools for investigating Orai3 function?

2-Aminoethoxydiphenyl borate (2-APB) is a particularly useful tool for studying Orai3 function due to its unique effects:

• Direct activation of Orai3: 2-APB (30-50 μM) can directly activate Orai3 channels independent of store depletion and even without STIM1 involvement

• Distinctive current profile: 2-APB-induced Orai3 currents display large inward and outward currents (double rectification) and a leftward shift in the reversal potential, indicating reduced Ca²⁺ selectivity and increased permeability to monovalent cations

• Differential effects on Orai isoforms: While 2-APB can also produce a similar but much smaller direct effect on Orai1 channels, the magnitude of response is significantly greater with Orai3, allowing pharmacological discrimination between channel types

For ARC channels containing Orai3, arachidonic acid serves as a specific activator, providing another tool to distinguish these channels from classical CRAC channels .

What structural features determine Orai3's unique functional properties?

Several key structural elements contribute to Orai3's distinctive properties:

N-terminal domain:

• Replacing the N-terminal cytosolic domain of Orai3 with the corresponding domain of Orai1 results in approximately doubling the magnitude of store-operated Ca²⁺ currents

• The critical region has been narrowed down to amino acids 42-62 in Orai3 (corresponding to 67-87 in Orai1)

• A specific lysine residue (K60 in Orai3, K85 in Orai1) is essential for activation

• N-terminal deletions to residues between W51 and Y55 can significantly increase store-operated Orai3-dependent currents

Pore-forming domains:

• Despite functional differences, the Ca²⁺ selectivity of Orai3 channels is similar to Orai1 channels

• This is consistent with the conservation of critical acidic residues in transmembrane domains 1 and 3 (E81 and E165 in Orai3, E106 and E190 in Orai1)

• In the extracellular loop between transmembrane domains 1 and 2, Orai3 has glutamates (E85, E89) in place of two of the three aspartates found in Orai1 (D110, D114)

What is the stoichiometry of Orai3 in different channel complexes?

Orai3 participates in different channel complexes with distinct stoichiometries:

In CRAC channels:

• Orai3 can form store-operated channels, though with reduced efficiency compared to Orai1

• When co-expressed with Orai1, Orai3 may form heteromeric CRAC channels with altered properties

In ARC channels:

• ARC channels display a unique heteropentameric stoichiometry comprising two Orai3 subunits and three Orai1 subunits

• This specific arrangement is essential for the channel's sensitivity to arachidonic acid

• Unlike CRAC channels, ARC channel activation relies exclusively on the pool of STIM1 that is constitutively resident in the plasma membrane

How does Orai3 contribute to B lymphocyte function?

Orai3 plays a significant role in B cell physiology:

• Expression dynamics: Orai3 expression changes in response to B cell activation. Stimulation with anti-IgM+anti-CD40 or LPS significantly increases Orai3 expression, while Orai2 expression is significantly reduced following stimulation conditions that increase mitochondrial respiration

• Functional redundancy: Deletion of Orai3 alone does not affect B cell function and humoral immune responses, suggesting that other Orai homologs (likely Orai1) can compensate for the loss of Orai3

• Combined function with Orai1: The combined loss of Orai1 and Orai3 (but not Orai3 alone) impairs:

  • Store-operated calcium entry (SOCE)

  • Proliferation and survival

  • Nuclear factor of activated T cells (NFAT) activation

  • Mitochondrial respiration and glycolysis

  • Metabolic reprogramming in response to antigenic stimulation

• In vivo immune response: Interestingly, the combined deletion of Orai1 and Orai3 in B cells did not compromise humoral immunity to influenza A virus infection in mice, suggesting that other in vivo co-stimulatory signals can overcome the requirement of BCR-mediated CRAC channel function

What methodologies are recommended for studying Orai3 in primary immune cells?

When investigating Orai3 in primary immune cells such as B lymphocytes, consider these specialized approaches:

B cell activation models:

• To study dynamic regulation of Orai3 expression, use different stimulation conditions:

  • Anti-IgM alone

  • Anti-CD40 alone

  • Combined anti-IgM+anti-CD40

  • Lipopolysaccharide (LPS)

Functional readouts:

• Calcium imaging or electrophysiology to measure SOCE

• Proliferation and survival assays

• Measurement of NFAT activation

• Assessment of mitochondrial respiration and glycolysis

• Metabolomic analysis to evaluate metabolic reprogramming

Genetic models:

• Single knockouts (Orai3-/-)

• Combined knockouts (Orai1-/- and Orai3-/-)

• Conditional knockouts to avoid developmental effects

• In vivo challenge models (e.g., influenza A virus infection) to assess physiological relevance

How does Orai3 contribute to store-independent calcium signaling?

Orai3 plays a critical role in store-independent calcium signaling through several mechanisms:

Direct activation by 2-APB:

• Orai3 can be directly activated by 2-APB (30-50 μM) independent of store depletion and even without STIM1 involvement

• This activation produces currents with distinctive properties:

  • Large inward and outward currents (double rectification)

  • Leftward shift in reversal potential

  • Reduced Ca²⁺ selectivity

  • Increased permeability to monovalent cations

ARC channel formation:

• Orai3 is an essential component of arachidonic acid-regulated calcium (ARC) channels

• These channels:

  • Require both Orai1 and Orai3 in a specific stoichiometry

  • Are activated by arachidonic acid independently of store depletion

  • Interact with the plasma membrane-resident pool of STIM1 rather than the ER-resident pool involved in SOCE

This dual involvement in both store-operated and store-independent calcium entry pathways makes Orai3 a versatile contributor to calcium signaling diversity in mammalian cells.

What are the implications of Orai3's evolutionary appearance specifically in mammals?

The evolutionary emergence of Orai3 specifically in mammals raises intriguing questions about its specialized functions:

• Orai3 is unique to mammals, apparently evolving from Orai1

• This evolutionary innovation has led to the development of conductances with novel properties—whether formed by Orai3 alone or in conjunction with other Orai proteins

These novel properties include:

• Formation of ARC channels with unique arachidonic acid sensitivity

• Direct activation by 2-APB independent of store depletion

• Contribution to B cell metabolic reprogramming during immune responses

The evolutionary appearance of Orai3 in mammals may be associated with its specific role in a unique range of distinct cellular activities, potentially contributing to specialized mammalian physiological processes .