Table 1.

The PI3K family

		Regulatory Subunit	Catalytic Subunit
CLASS I	Ia	p85α, p85β, p55γ	p110α, p110β, p110δ
	Ib	p101	p110γ
CLASS II		?	PI3KC2α, PI3KC2β
CLASS III		p150	Vps34p homologue

The table shows the regulatory and catalytic subunits of the three classes of PI3K. The heterodimeric class Ia PI3K signal downstream of tyrosine kinases and Ras. The p85α regulatory subunit may generate p55α and p50α by alternative splicing. Class Ib PI3K signal downstream of GPCR and Ras. There is little information on the mechanism of activation for class II PI3K, although PI3KC2β has been implicated in lysophosphatidic acid-mediated migration of mesenchymal-like cells. The class III PI3K uses unphosphorylated phosphatidylinositol as a substrate to produce PI₃P.