RhoGEFs, Kalirin and Trio, the mammalian homologs of Drosophila Trio and Caenorhabditis elegans UNC-73 regulate a novel step in secretory granule maturation. Their signaling modulates the extent to which regulated cargo enter and remain in the regulated secretory pathway. This allows for fine tuning of peptides released by a single secretory cell type with impaired signaling leading to pathological states. Trio plays an essential role in regulating the actin cytoskeleton during axonal guidance and branching. Kalirin and Trio are encoded by separate genes in mammals and by a single one in invertebrates. Kalirin and Trio share the same complex multidomain structure and display several splice variants. The longest Kalirin and Trio proteins have a Sec14 domain, a stretch of spectrin repeats, a RhoGEF(DH)/PH cassette (also called GEF1), an SH3 domain, a second RhoGEF(DH)/PH cassette (also called GEF2), a second SH3 domain, Ig/FNIII domains, and a kinase domain. The first RhoGEF(DH)/PH cassette catalyzes exchange on Rac1 and RhoG while the second RhoGEF(DH)/PH cassette is specific for RhoA. Kalirin and Trio are closely related to p63RhoGEF and have PH domains of similar function. 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.
Jiyao W et al.(2023). "The conserved domain database in 2023.",