Cloning of ASH, a ubiquitous protein composed of one Src homology region (SH) 2 and two SH3 domains, from human and rat cDNA libraries.

The protein ASH (for abundant Src homology), composed of one Src homology region (SH) 2 and two SH3 domains, was cloned by screening human and rat cDNA libraries with an oligonucleotide probe directed to a consensus sequence of the SH2 domains. The rat-derived ASH peptide was comprised of 217 amino acids with a molecular mass of 25-28 kDa and was found to be ubiquitous in rat tissues. A human cDNA clone was also found to code for part of the same protein, suggesting that ASH is common to human and rat. The amino acid sequence of ASH was strikingly similar to Sem-5, the product of a nematode cell-signaling gene, and ASH is most probably a mammalian homologue of Sem-5. ASH bound in vitro to phosphotyrosine-containing proteins, including activated epidermal growth factor receptor, the ASH SH2 domain being responsible for the binding. Induced expression of an antisense ASH cDNA led to a reduction in cell growth. Considering these observations and the structural homology to Sem-5, ASH is likely to function as a ubiquitous signal transducer, possibly resembling Sem-5, which communicates between a receptor protein tyrosine kinase and a Ras protein.