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Author(s): R. X. Zhu (corresponding author) [1]; R. Potts (corresponding author) [2]; F. Xie [3]; K. A. Hoffman [4]; C. L. Deng [1]; C. D. Shi [1]; Y. X. Pan [...]

The growth suppression function of the retinoblastoma protein (Rb) is though to be mediated by Rb binding to cellular proteins. p48 is one of the major proteins that binds to a putative functional domain at the carboxy terminus of the Rb protein. Here we report the isolation of a full-length complementary DNA (RbAp48) encoding p48. Complex formation between p48 and Rb occurs in vitro and in vivo, and apparently involves direct interaction between the proteins. Like Rb, p48 is a ubiquitously expressed nuclear protein. RbAp48 share sequence homology with MSI1, a negative regulator of the Ras-cyclic AMP pathway in the yeast Saccharomyces cerevisiae. Furthermore, like MSI1, human RbAp48 suppresses the heat-shock sensitivity of the yeast ira1 strains and RAS2Val19 strains. Interaction with p48 may be one of the mechanisms for suppression of growth mediated by Rb.

The retinoblastoma gene, a prototypic tumour-suppressor gene, encodes a nuclear phosphoprotein (Rb). To understand better the role of Rb in development and in tumorigenesis, mice with an insertional mutation in exon 20 of the Rb-1 locus were generated. Homozygous mutants die before the 16th embryonic day with multiple defects. The haematopoietic system is abnormal; there is a significant increase in the number of immature nucleated erythrocytes. In the nervous system, ectopic mitoses and massive cell death are found, particularly in the hindbrain. All spinal ganglion cells die, but the neural retina is unaffected. Transfer of the human retinoblastoma (RB) mini-transgene into the mutant mice corrects the developmental defects. Thus, Rb is essential for normal mouse development.