Budding yeast contain two main p21-activated kinases (PAKs), Cla4 and Ste20. The yeast Ste20 protein kinase is involved in pheromone response, though the function of Ste20 mammalian homologs is unknown. Cla4 is involved in budding and cytokinesis and interacts with Cdc42, a GTPase required for polarized cell growth as is Pak. Cla4 and Ste20 kinases share a function in localizing cell growth with respect to the septin ring. They both contain a PH domain, a Cdc42/Rac interactive binding (CRIB) domain, and a C-terminal Protein Kinase catalytic (PKc) domain. PH domains have diverse functions, but in general are involved in targeting proteins to the appropriate cellular location or in the interaction with a binding partner. They share little sequence conservation, but all have a common fold, which is electrostatically polarized. Less than 10% of PH domains bind phosphoinositide phosphates (PIPs) with high affinity and specificity. PH domains are distinguished from other PIP-binding domains by their specific high-affinity binding to PIPs with two vicinal phosphate groups: PtdIns(3,4)P2, PtdIns(4,5)P2 or PtdIns(3,4,5)P3 which results in targeting some PH domain proteins to the plasma membrane. A few display strong specificity in lipid binding. Any specificity is usually determined by loop regions or insertions in the N-terminus of the domain, which are not conserved across all PH domains. PH domains are found in cellular signaling proteins such as serine/threonine kinase, tyrosine kinases, regulators of G-proteins, endocytotic GTPases, adaptors, as well as cytoskeletal associated molecules and in lipid associated enzymes.
Jiyao W et al.(2023). "The conserved domain database in 2023.",