Identification of a human brain-specific isoform of mammalian STE20-like kinase 3 that is regulated by cAMP-dependent protein kinase

A novel isoform of mammalian STE20-like kinase 3 (MST3) with a different 5' coding region from MST3, termed MST3b, was identified by searching through expressed sequence tag data base and obtained by rapid amplification of cD NA 5'-ends, MST3b was assigned to the long arm of human chromosome 13, D13S 159-D13S280, by use of the National Center for Biotechnology Information se quence-tagged sites data base. Reverse transcription-polymerase chain react ion and Northern blot analysis with a probe derived from 5' distinct sequen ce of MST3b revealed that the expression of MST3b mRNA is restricted to the brain, in contrast to ubiquitous distribution of MST3 transcript. Western analysis confirmed the brain-specific expression of MST3b protein. In situ hybridization of rat brain sections with a MST3b-specific probe indicated t hat MST3b is widely expressed in different brain regions, with especially h igh expression in hippocampus and cerebral cortex. When expressed in human embryonic kidney 293 (HEK293) cells, MST3b effectively phosphorylated myeli n basic protein, as well as undergoing autophosphorylation, Interestingly, expression of MST3, but not MST3b, in HEK293 cells was able to activate the endogenous p42/44 mitogen-activated protein kinase (MAPK) up to 4-fold, wh ereas neither isoform activated p38 MAPK under the same conditions. Further experiments demonstrated that MST3b, but not MST3, was effectively phospho rylated by activation of cyclic AMP-dependent protein kinase (PKA) in both in vivo and in vitro assays. The mutation of Thr-18 into Ala in MST3b (T18A ), a putative PHA phosphorylation site that is absent in MST3, abolished it s phosphorylation by PKA, Consequently, expression of the T18A mutant in HE K293 cells led to partial activation of p42/44 MAPK, indicating that MST3b is under the regulation of PKA, Taken together, our data provide evidence t hat the two isoforms of STE20-like kinase 3 are differentially distributed and regulated.