ISOLATION OF MEK5 AND DIFFERENTIAL EXPRESSION OF ALTERNATIVELY SPLICED FORMS

The prototype mitogen-activated protein (MAP) kinase module is a three -kinase cascade consisting of the MAP kinase, extracellular signal-reg ulated protein kinase (ERK) 1 or ERK2, the MAP/ERK kinase (MEK) MEK1 o r MEK2, and the MEK kinase, Raf-1 or B-Raf. This and other MAP kinase modules are thought to be critical signal transducers in major cellula r events including proliferation, differentiation, and stress response s. To identify novel mammalian MAP kinase modules, polymerase chain re action was used to isolate a new MEK family member, MEK5, from the rat . MEK5 is more closely related to MEK1 and MEK2 than to the other know n mammalian MEKs, MKK3 and MKK4. MEK5 is thought to lie in an uncharac terized MAP kinase pathway, because MEK5 does not phosphorylate the ER K/MAP kinase family members ERK1, ERK2, ERK3, JNK/SAPK, or p38/HOG1, n or will Raf-1, c-Mos, or MEKK1 highly phosphorylate it. Alternative sp licing results in a 50-kDa alpha and a 40-kDa beta isoform of MEK5. ME K5 beta is ubiquitously distributed and primarily cytosolic. MEK5 alph a is expressed most highly in liver and brain and is particulate. The 23 amino acids encoded by the 5' exon in the larger alpha isoform are similar to a sequence found in certain proteins believed to associate with the actin cytoskeleton; this alternatively spliced modular domain may lead to the differential subcellular localization of MEK5 alpha.