Deletion of the COOH terminus converts the ST5 p70 protein from an inhibitor of RAS signaling to an activator with transforming activity in NIH-3T3 cells.

Expression of the human protein ST5-p70 correlates with reduced tumorigenic phenotype in mammalian cells, reverts their transformed phenotype, and restores their contact-dependent growth. Furthermore, expression of p70 in COS-7 cells suppresses activation of mitogen activated protein kinase MAPK/ERK2 by the largest ST5 product, p126, in response to epidermal growth factor stimulation. Here we show that deletions of the COOH-terminal region of p70 transform NIH3T3 cells and induce their anchorage-independent growth. Analysis of signaling leading to MAPK/ERK2 stimulation revealed that in COS-7 cells, expression of either p70-DeltaC1 or p70-DeltaC2 markedly enhanced ERK2 activity in a growth factor-independent manner. Whereas wild-type p70 slightly inhibited ERK2 activation by RAS and MEK2, co-expression or p70-DeltaC1 or p70-DeltaC2 with either protein stimulated ERK2 cooperatively. This activity was completely blocked by the dominant negative mutants RAS17N or MEKAA, suggesting that p70 functions upstream of RAS. Unlike wild-type p70, expression of p70-DeltaC1 or p70-DeltaC2 mutant did not interfere with the ability of ST5-p126 to stimulate ERK2. Taken together, the data suggest that the COOH-terminal tail, residues 489-609, contains some of the critical determinants for the function of p70. Loss of this region converts the protein from an inhibitor to a constitutive activator of the RAS-ERK2 pathway.