p85

P85 refers to the regulatory subunits of phosphoinositide 3-kinase (PI3K), most commonly discussed isoforms being p85α, p85β, and p55γ in humans. These subunits are encoded by the genes PIK3R1, PIK3R2, and PIK3R3, respectively. The p85 proteins form heterodimers with the catalytic subunits of PI3K (p110 α, β, or δ) and influence PI3K activity through interactions with phosphotyrosine motifs and other signaling partners.

Functionally, p85 subunits contain SH2 domains that bind phosphorylated tyrosine residues on activated receptors or adaptor proteins. This binding recruits the p110 catalytic subunit to the plasma membrane, enabling the production of the second messenger PIP3. PIP3 then activates downstream kinases such as AKT, integrating signals related to growth, survival, metabolism, and angiogenesis. The regulatory subunits also exert autoinhibitory control over p110 in the absence of activating signals, helping to fine-tune PI3K signaling.

Clinical and biological significance is strong, as dysregulation of the PI3K p85/p110 axis can contribute to disease. Mutations or altered expression of PIK3R1 (p85α) have been linked to developmental disorders such as SHORT syndrome, and somatic alterations in PIK3R2 or PIK3R3 have been observed in various cancers, where they can modify pathway activity and cellular behavior.

In naming conventions, the “p” stands for protein, and the numerical designation differentiates isoforms. The p85 regulatory subunits are central to canonical PI3K signaling and its role in cell physiology.