Cloning and Initial Functional Characterization of Mlk4α and Mlk4

We have cloned a novel human mixed-lineage kinase gene, MLK4. Two alternatively spliced forms, MLK4α (580 aa) and MLK4 (1036 aa), have been identified and mapped to chromosomal band 1q42. MLK4 shows high amino acid homology to the kinase catalytic domain of MLK3 (72%), MLK1 (71%) and MLK2 (69%). Strong expression of MLK4 was detected in the human pancreas and kidneys. pCMV-MLK4 c-myc-tagged protein (human) was expressed in the cytoplasm and nucleus of transiently transfected COS-1 cells, while pCMV-MLK4α c-myc-tagged protein (human) was expressed in cytoplasm only. Both MLK4 isoforms reduced the colony formation ability of MCF7 cells by 85%–95% and almost totally suppressed cell proliferation in the CyQUANT cell proliferation assay. Human pCMV-MLK4 transgenic mice expressed the MLK4 in all tissues examined but no phenotypic abnormalities were observed. Thus, in this work, we present the cloning and sequencing of MLK4α and MLK4 for the first time; the data obtained suggest that MLK4 may function as a MAP kinase.