A novel Akt/PKB-related kinase is essential for morphogenesis in Dictyostelium

Background: Dictyostelium Akt/PKB is homologous to mammalian Akt/PKB and is required for cell polarity and proper chemotaxis during early development. The kinase activity of Akt/PKB kinase is activated in response to chemoatt ractants in neutrophils and in Dictyostelium by the chemoattractant cAMP fu nctioning via a pathway involving a heterotrimeric G protein and PI3-kinase . Dictyostelium contains several kinases structurally related to Akt/PKB, o ne of which, PKBR-1, is investigated here for its role in cell polarity, mo vement and cellular morphogenesis during development. Results: PKBR-1 has a kinase and a carboxy-terminal domain related to those of Akt/PKB, but no PH domain. Instead, it has an amino-terminal myristoyla tion site, which is required for its constitutive membrane localization. Li ke Akt/PKB, PKBR-1 is activated by cAMP through a G-protein-dependent pathw ay, but does not require PI3-kinase, probably because of the constitutive m embrane localization of PKBR-1. This is supported by experiments demonstrat ing the requirement for membrane association for activation and in vivo fun ction of PKBR-1. PKBR-1 protein is found in all cells throughout early deve lopment but is then restricted to the apical cells in developing aggregates , which are thought to control morphogenesis. PKBR-1 null cells arrest deve lopment at the mound stage and are defective in morphogenesis and multicell ular development. These phenotypes are complemented by Akt/PKB, suggesting functional overlap between PKBR-1 and Akt/PKB. Akt/PKB PKBR-1 double knocko ut cells exhibit growth defects and show stronger chemotaxis and cell-polar ity defects than Akt/PKB null cells. Conclusions: Our results expand the previously known functions of Akt/PKB f amily members in cell movement and morphogenesis during Dictyostelium multi cellular development. The results suggest that Akt/PKB and PKBR-1 have over lapping effecters and biological function: Akt/PKB functions predominantly during aggregation to control cell polarity and chemotaxis, whereas PKBR-1 is required for morphogenesis during multicellular development. (C) 2000 El sevier Science Ltd. All rights reserved.