beta TrCP plays a relevant role in the control of stability of several key protein factors. In Xenopus, beta TrCP acts as an inhibitor of Wnt signaling and dorsal axis formation. We determined the primary structure of the frog beta TrCP gene, which consists of 14 exons and 13 introns, spanning over 34 kb. Isoforms of x-beta TrCP have been found, which show differences in the NH(2) and COOH regions. NH(2) isoforms differ for the presence or absence of a 30 aa sequence, coded by exon III. In COOH isoforms, 19 C-terminal amino acids are replaced by three different amino acids. Occurrence of two 5' splice donor sites for splicing of intron XIII provides an explanation for these isoforms, based on alternative splicing. The DNA region of the putative beta TrCP promoter contains several TATA elements, one GCCAAT box, and putative binding sites for Ets, Tcf/Lef and NF-kappa B transcription factors. Two transcription initiation sites have been mapped downstream of TATA boxes proximal to ATG for start of translation. Comparison of the Xenopus and human beta TrCP genes indicates high conservation of exon nucleotide and amino acid sequences, size and organization; differences are limited to exons coding for N- and C-terminal regions.