Cellular Functions of Oncoproteins
Oncoproteins and their corresponding proto-oricoproteins are variously distributed in the nucleus, cytoplasm and the nuclear membrane. On the cell or nuclear membrane, they may be found on the cytoplasmic face or may be found as transmembrane proteins. Distribution of a number of oncoproteins is shown in Figure 48.8. They are also classified, on the basis of function into following four groups : (i) tyrosine kinases (subdivided into cytoplasmic proteins and membrane-bound growth factor receptors), (ii) growth factors, (iii) GTP-binding proteins and (iv) nuclear proteins.

Src (associated with RSV) was the first oncoprotein of the kinase type, that was characterized. Src is located on the cytoplasmic face of the cell membrane (Fig. 45.8). In the cells transformed by RSV, phosphotyrosine is increased ten times, and Src is responsible not only for its own phosphorylation (at residues 416, 527) but also for phosphorylation of tyrosine in several other proteins. Its catalytic activity resides in the c-terminal half of the protein and this activity in v-Src is 20 times that in c-Src. The oncogenic property of Src also depends on the replacement of N-terminal amino acid of Src by an unusual amino acid 'myristic acid', a phenomenon described as myristylation. How does phosphorylation of c-Src and v-Src regulates cellular function is being studied in some detail.