An avian cDNA encoding a tyrosine-phosphorylated protein with PDZ, coiled-coil, and SAM domains

Salpalpha and Salpbeta, growth-arresting homologs of Sam68

Sam68, a nuclear RNA-binding protein, is a major substrate of the Src tyrosine kinase in mitotic cells. In addition to a tyrosine-rich C-terminal region, Sam68 also has six poly-proline (SH3-binding) sites, many of which are located in an amino-terminal region. Sam68 appears to act as an adaptor protein, associating with many SH2- and SH3-containing signal-transducing proteins (Richard et al., Mol. Cell. Biol. 15:186-197, 1995). Here we describe a novel 55kDa protein, Salpalpha, which has sequence similarity to Sam68 throughout its length. Salpalpha lacks the amino-terminal region found in Sam68, and has only a single poly-proline site, which binds the SH3 domain of the p85 subunit of PI 3-kinase. Salpalpha is tyrosine-phosphorylated when expressed in Rous sarcoma virus-infected chicken embryo fibroblasts (RSV-CEF); unlike Sam68, however, Salpalpha does not co-precipitate with v-Src. Salpbeta, an alternatively spliced isoform lacking the C-terminal tyrosine-rich region, is also tyrosine-phosphorylated in RSV-CEF, and also binds the SH3 domain of p85. We further show that expression of either Salpalpha or Salpbeta down-regulates the expression of Sam68 in CEF, and arrests the growth of these cells. Our results suggest that Salp may function as a negative regulator of cell growth.

Bau, a splice form of Neurabin-I that interacts with the tumor suppressor Bin1, inhibits malignant cell transformation

Bin1 is a nucleocytoplasmic adaptor protein and tumor suppressor. A novel protein termed Bau was identified through its ability to interact with a region of Bin1 required to inhibit malignant cell transformation by certain oncogenes. Bau is a splice form of Neurabin-I, one of two related F-actin-binding proteins that are proposed to link cadherin-based cell-cell adhesion sites with the growth regulatory kinase p70S6K. Bau lacks actin- and p70S6K-binding domains found in Neurabin-I but includes coiled-coil domains that are part of its central domain as well as additional sequences not found in Neurabin-I. Interaction with Bin1 requires the presence of the U3 region which is alternately spliced in muscle cells. Bau localizes to the nucleus and cytosol. Like Bin1, Bau can suppress oncogene-mediated transformation and inhibit tumor cell growth. We suggest that Bau may link Bin1 to the Neurabin-I/p70S6K system in muscle and other cells, perhaps providing a mechanism to influence adhesion-dependent signals which affect cell fate.