AN ALTERNATIVELY SPLICED MESSENGER-RNA FROM THE AP-2 GENE ENCODES A NEGATIVE REGULATOR OF TRANSCRIPTIONAL ACTIVATION BY AP-2

AP-2 is a retinoic acid-inducible and developmentally regulated activa tor of transcription. We have cloned an alternative AP-2 transcript (A P-2B) from the human teratocarcinoma cell line PA-1, which encodes a p rotein differing in the C terminus from the previously isolated AP-2 p rotein (AP-2A). This protein contains the activation domain of AP-2 an d part of the DNA binding domain but lacks the dimerization domain whi ch is necessary for DNA binding. Analysis of overlapping genomic clone s spanning the entire AP-2 gene proves that AP-2A and AP-2B transcript s are alternatively spliced from the same gene. Both transient and sta ble transfection experiments show that A.P-2B inhibits AP-2 transactiv ator function, as measured by an AP-2-responsive chloramphenicol acety ltransferase reporter plasmid. Furthermore, constitutive AP-2B express ion in PA-1 cells causes a retinoic acid-resistant phenotype, anchorag e-independent growth in soft agar, and tumorigenicity in nude mice, in a fashion similar to transformation of these cells by oncogenes. To d etermine the mechanism by which A.P-2B exerts its inhibitory function, we purified bacterially expressed AP-2A and AP-2B proteins. While bac terial AP-2B does not bind an A.P-2 consensus site, it strongly inhibi ts binding of the endogenous AP-2 present in PA-1 cell nuclear extract s. However, DNA sequence-specific binding of bacterially expressed AP- 2A cannot be inhibited by bacterially expressed AP-2B. Therefore, inhi bition of AP-2 activity by the protein AP-2B may require an additional factor or modification supplied by nuclear extracts.