Cloning of a novel human Rac1b splice variant with increased expression incolorectal tumors

Rac1 is a member of the Ras superfamily of small GTPases involved in signal transduction pathways that induce the formation of lamellipodia, stimulate cell proliferation and activate the JNK/SAPK protein kinase cascade, Here we describe that amplification by RT-PCR of the entire Rac1 coding sequence from a series of human adult and fetal tissues revealed beside the expecte d Rac1 cDNA, a variant product which contained additional 57 nucleotides be tween codons 75 and 76. This variant resulted in an in-frame insertion of 1 9 new amino acids immediately behind the switch II region, including two po tential threonine phosphorylation sites for casein kinase II and protein ki nase C, Primers designed within and downstream of the inserted nucleotide s equence allowed isolation of a genomic clone with intronic consensus sequen ces demonstrating that the insertion corresponds to a novel, yet undescribe d exon 3b, This Rac1 splice variant, designated Rac1b, was predominantly id entified in skin and epithelial tissues from the intestinal tract. Most not ably, the expression of rac1b versus rad was found to be elevated in colore ctal tumors at various stages of neoplastic progression, as compared to the ir respective adjacent tissues. We suggest that the 19 amino acid-insertion following the switch II region may create a novel effector binding site in rac1b, and thus participate in signaling pathways related to the normal or neoplastic growth of the intestinal mucosa.