betaPix (Pak-interacting exchange factor) isoforms are recently identified guanine nucleotide exchnge factors (GEFs) for Rho family GTPases. Rac/Cdc42, that are key players in the regulation of actin dynamics. Here we show that a novel 105-kDa betaPix isoform, betaPix-b(L), is generated by alternative translation of betaPix-b mRNA. Translation of betaPix-b(L) starts at an atypical initiation site, GTG, that is located 57 nucleotides downstream from the newly identified 5' end of betaPix-b cDNA. The expression of two isoforms, betaPix-b and betaPix-b(L). from betaPix-b mRNA is controlled by an internal ribosome entry site (IRES)-driven mechanisin. Comparing to betaPix-b, betaPix-b(L) Contains additional 105 amino acids composed of a calponin homology (CH) domain and a serine-rich sequence in the N-terminus. The expression of betaPix-b(L) in rat brain is developmentally regulated and high in the embryonic stages, suggesting that the function of betaPix-b(L) is more heavily required during the early stages of brain development. (C) 2004 Elsevier Inc. All rights reserved.