The phosphotyrosine-binding (PTB) domain of Numb, a protein involved in asymmetric cell division, has recently been shown to bind to the adapter protein Lnx through an LDNPAY sequence, to the Numb-associated kinase (Nak) through a sequence that does not contain an NPXY motif and to GP(p)Y-containing peptides obtained from library screening. We show here that these diverse peptide sequences bind with comparable affinities to the Numb PTB domain at a common binding site on the surface of the protein. The NMR structure of the Numb PTB domain in complex with a GPpY-containing peptide reveals a novel mechanism of binding with the peptide in a helical turn that does not hydrogen bond to the PTB domain beta-sheet. These results suggest that PTB domains can potentially have multiple modes of peptide recognition and provide a structural basis from which the multiple functions of the Numb PTB domain during asymmetric cell division could arise.

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The specific contributions of human adenovirus type 5 early region 1B (E1B) proteins were examined using mutants which synthesize these products individually. In cooperation with E1A, transformation of primary baby rat kidney cells was achieved with either the 176R protein or 496R protein alone, albeit at an efficiency considerably less than that observed when both were present. These results indicate that transformation mediated by either E1B product can proceed independently, but that the processes involved are additive.

Deregulated alternative splicing of the endocytic adaptor NUMB resulting in high expression of Exon9in (exon 9-containing) isoforms has been reported in several cancer types. However, the role of Numb isoform expression in tumor progression and the underlying mechanisms remain elusive. Here, we report greater exon 9 inclusion in multiple cancer types including all subtypes of breast cancer, and correlation of higher exon 9 inclusion in patients with worse prognosis. Deletion of Exon9in in breast cancer cells leads to reduced cell growth and a significant decrease of lung metastasis in orthotopic xenograft experiments. Quantitative mass spectrometry revealed downregulation of proteins involved in EMT and ECM organization and remodeling of the endocytic protein network in cells lacking the Exon9in Numb isoforms. Exon 9 deletion also results in reduced surface levels of ITGβ5, and downstream signaling to ERK and SRC, consistent with enhance lysosomal targeting mediated by the remaining Exon9sk (exon 9 skipping) Numb isoforms. Our findings reveal that Exon9in isoforms promote breast cancer progression by relieving Numb mediated down regulation of integrins and implicate Numb alternative splicing as a progression factor in multiple cancer types.