TABLE 1.

Similarities and differences between the type I and type II PAKs

	Type I PAKs (PAK1, PAK2, PAK3)	Type II PAKs (PAK4, PAK5, PAK6)
GTPase binding	Yes, via CRIB domain	Yes, via CRIB domain
GTPase specificity	RAC ≈ CDC42	CDC42 > RAC
Direct activation by GTPase	Yes	Noa
Autoinhibition mechanism	Trans-autoinhibited homodimer	Pseudosubstrate autoinhibition
Autoinhibited stoichiometry in solution	Homodimeric	Monomeric
Activation loop phosphorylation state (autoinhibited)	Unphosphorylated	Phosphorylated
Activation mechanism	GTPase binding-associated conformational change; activation loop phosphorylation	Pseudosubstrate release
Phosphoacceptor residue specificity	Ser > Thr	Ser > Thr

^a Most studies indicate that the type II PAKs are not directly activated by small GTPase binding.