Serine/threonine-protein kinase PAK 5 is an enzyme that in humans is encoded by the PAK5 gene. The PAK5 enzyme is one of three members of Group II PAK family of serine/threonine kinases, and evolutionary conserved across species. Discovery The PAK5 was initially cloned as a brain-specific kinase with a predominant expression in brain with a suggested role in neurite growth in neuronal cells. Selectivity of PAK5 signaling was recognized by its ability to stimulate the JNK kinase but not p38 or ERK kinases. Gene and spliced variants The PAK5 gene, the longest among the PAK family, contains a total of 12 exons of which four exons are for 5’-UTR and remaining 8 exons for protein coding(Gene from review). Alternative exon splicing of the PAK5 gene generates three transcripts, and one of the transcript encodes a 719 amino acids long protein(Gene from review). The exon splicing of the murine PAK5 gene generates three transcripts, two of which code an identical 719 amino acids long polypeptide while the 2.0-kb transcript is a non-coding RNA with retained intron. Protein domains Similar to PAK4, PAK5 consists of a kinase, a CDC42/Rac1 interactive binding (CRIB) motif. Function The protein encoded by this gene is a member of the PAK family of Ser/Thr protein kinases. PAK family members are known to be effectors of Rac/Cdc42 GTPases, which have been implicated in the regulation of cytoskeletal dynamics, proliferation, and cell survival signaling. This kinase contains a CDC42/Rac1 interactive binding (CRIB) motif, and has been shown to bind CDC42 in the presence of GTP. This kinase is predominantly expressed in brain. It is capable of promoting neurite outgrowth, and thus may play a role in neurite development. This kinase is associated with microtubule networks and induces microtubule stabilization. The subcellular localization of this kinase is tightly regulated during cell cycle progression. Alternatively spliced transcript variants encoding the same protein have been described. Genetic deletion of PAK5 with or without PAK6 deletion in mice has been shown to be associated with a defective locomotion, memory, and learning. PAK5 is co-expressed with DISC1, a psychosis risk gene, and the pathway is likely to be involved in modulating synapse plasticity. Physiological level of PAK5 is linked with an overall physical activity in mice as PAK5 deletion in mice has been shown to be associated with an increased activity upon amphetamine stimulation. PAK5 has been also thought to be one of genetic