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 chemoattractants in neutrophils and in Dictyostelium by the chemoattractant cAMP functioning via a pathway involving a heterotrimeric G protein and PI3-kinase. Dictyostelium contains several kinases structurally related to Akt/PKB, one of which, PKBR-1, is investigated here for its role in cell polarity, movement 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 myristoylation site, which is required for its constitutive membrane localization. Like Akt/PKB, PKBR-1 is activated by cAMP through a G-protein-dependent pathway, but does not require PI3-kinase, probably because of the constitutive membrane localization of PKBR-1. This is supported by experiments demonstrating the requirement for membrane association for activation and in vivo function of PKBR-1. PKBR-1 protein is found in all cells throughout early development but is then restricted to the apical cells in developing aggregates, which are thought to control morphogenesis. PKBR-1 null cells arrest development at the mound stage and are defective in morphogenesis and multicellular development. These phenotypes are complemented by Akt/PKB, suggesting functional overlap between PKBR-1 and Akt/PKB. Akt/PKB PKBR-1 double knockout cells exhibit growth defects and show stronger chemotaxis and cell-polarity defects than Akt/PKB null cells.


CONCLUSIONS
Our results expand the previously known functions of Akt/PKB family members in cell movement and morphogenesis during Dictyostelium multicellular development. The results suggest that Akt/PKB and PKBR-1 have overlapping effectors 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.
Meili
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Section of Cell and Development Biology, Center for Molecular Genetics, University of California, La Jolla 92093-0634, USA.
Ellsworth
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Firtel
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D000595:Amino Acid Sequence; D000818:Animals; D003001:Cloning, Molecular; D018076:DNA, Complementary; D004023:Dictyostelium / Q000201:enzymology* / Q000254:growth & development*; D018507:Gene Expression Regulation, Developmental; D015971:Gene Expression Regulation, Enzymologic; D005816:Genetic Complementation Test; D008969:Molecular Sequence Data; D009024:Morphogenesis; D017346:Protein Serine-Threonine Kinases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D015800:Protozoan Proteins; D017386:Sequence Homology, Amino Acid
D018076:DNA, Complementary; D015800:Protozoan Proteins; C411869:PKBR-1 protein, Dictyostelium; D017346:Protein Serine-Threonine Kinases