Cloning and characterization of a testis and brain-specific isoform of mouse 3'-phosphoinositide-dependent protein kinase-1 mPDK-1β

3'-Phosphoinositide-dependent protein kinase-1 (PDK-1) phosphorylates and activates members of the protein kinase AGC family and plays a key role in receptor tyrosine kinase signaling. Here we report the cloning and characterization of a splice variant of mouse PDK-1 mPDK-1β. The cDNA encoding mPDK-1β contains two alternative start codons and translation from these start codons generates proteins that are respectively 27 or 51 amino acid residues shorter at the amino-terminus than the previously identified PDK-1 isolated from mouse liver (now renamed mPDK-1α) [J. Biol. Chem. 274 (1999) 8117]. Analysis of mouse tissues shows that mPDK-1β is highly expressed in the testis and various functional regions of the brain. Expression of this isoform is increased in the brain of aged mice. Both mPDK-1α and mPDK-1β are autophosphorylated at both serine and threonine residues in vitro and showed similar levels of tyrosine phosphorylation when co-expressed with either constitutively active Src or Fyn tyrosine kinases in cells. However the mPDK-1 isoforms showed significant differences in their response to pervanadate- or insulin plus vanadate-stimulated tyrosine phosphorylation. Taken together our findings suggest that the two PDK-1 isoforms may be differentially regulated in cells. The specific expression of mPDK-1β in mouse testis and brains of aged mice also suggests potential involvement of this kinase in regulating animal spermatogenesis and aging.