Mitogen-activated protein kinase kinase kinase kinase 5 is a member of the serine/threonine protein kinase family, that is highly similar to yeast SPS1/STE20 kinase. Yeast SPS1/STE20 functions near the beginning of the MAP kinase signal cascades that is essential for yeast pheromone response. This kinase was shown to activate Jun kinase in mammalian cells, which suggested a role in stress response.

Northern blot analysis detected a 4.5-kb MAP4K5 transcript in all tissues tested, namely spleen, thymus, prostate, testis, ovary, small intestine, colon, and peripheral blood leukocytes. SDS-PAGE and immunoblot analyses determined that MAP4K5 is expressed as a 95-kD protein. Confocal laser microscopy demonstrated cytoplasmic expression of MAP4K5.