Splicing isoforms of rat Ash/Grb2. Isolation and characterization of the cDNA and genomic DNA clones and implications for the physiological roles of the isoforms.
We obtained three types of cDNA clones homologous to Ash/Grb2(Ash-l) cDNA from rats. One of these clones, Ash-psi, was an unusual transcribed gene having 93% identity in the nucleotide sequence to Ash-l. The other two clones, Ash-m and -s, had nucleotide sequences identical with Ash-l cDNA in the amino-terminal region. The coding sequence of Ash-m cDNA is 42 nucleotides shorter than that of Ash-l cDNA. The defective region of Ash-m cDNA encodes 14 amino acid residues (157 to 170 of Ash-l), which comprise the most conserved region of the second SH3 domain. On the other hand, the coding sequence of Ash-s terminated at the end of the first SH3 domain due to a stop codon at the boundary of the sequence, thereby differing from Ash-l cDNA. Cloning of the genomic DNA of the Ash-l-encoding gene, determination of the gene organization, and nucleotide sequencing revealed that the two isoforms, as well as Ash-l, are generated from a single gene by unusual alternative splicings. The gene spans more than 16 kilobases and contains 6 exons and 5 introns. Ash-m and Ash-s mRNAs were detected in various tissues by reverse-transcribed polymerase chain reaction. Ash-m physically associated with dynamin, but the association with Sos was less effective than that of Ash-l in rat pheochromocytoma PC12 cell lysates, irrespective of treatment with nerve growth factor. In contrast, Ash-s formed a complex with dynamin and Sos in cell lysates. Moreover, the newly formed carboxyl-terminal SH3 of Ash-m by splicing bound different proteins from those bound to the carboxyl-terminal SH3 domain of Ash-l, suggesting that Ash-m generates different signals. Microinjection of Ash-m or Ash-s into Balb/c 3T3 cells inhibited DNA synthesis induced by platelet-derived growth factor. These results show that these isoforms act as dominant negative regulators of mitogenic signals by Ash-l.