Nucleotide sequence of the src gene of the Schmidt-Ruppin strain of Rous sarcoma virus type E

Conformational processing of oncogenic v-Src kinase by the molecular chaperone Hsp90

Hsp90 is a molecular chaperone involved in the activation of numerous client proteins, including many kinases. The most stringent kinase client is the oncogenic kinase v-Src. To elucidate how Hsp90 chaperones kinases, we reconstituted v-Src kinase chaperoning in vitro and show that its activation is ATP-dependent, with the cochaperone Cdc37 increasing the efficiency. Consistent with in vivo results, we find that Hsp90 does not influence the almost identical c-Src kinase. To explain these findings, we designed Src kinase chimeras that gradually transform c-Src into v-Src and show that their Hsp90 dependence correlates with compactness and folding cooperativity. Molecular dynamics simulations and hydrogen/deuterium exchange of Hsp90-dependent Src kinase variants further reveal increased transitions between inactive and active states and exposure of specific kinase regions. Thus, Hsp90 shifts an ensemble of conformations of v-Src toward high activity states that would otherwise be metastable and poorly populated.

Transformation-defective mutants with 5' deletions of the src gene are frequently generated during replication of Rous sarcoma virus in established quail fibroblasts

Replication of Rous sarcoma virus (RSV) in avian fibroblasts leads to the generation of replication-competent variants that are defective for cell transformation (td virus). These td variants contain deletions affecting various portions of the v-src gene. We compared the rate of td virus production in Q3B cells, a quail cell line established by mutagen treatment, and in normal quail fibroblasts. Twenty-five days after infection with an RSV stock containing only transforming virions, Q3B cells harbor similar amounts of v-src-containing and v-src-deleted proviruses. However, these cells synthesize very low levels of p60v-src and generate large excess of td variants, as determined by biological assays. Unlike Q3B cells, normal quail fibroblasts infected with the same virus stock produce td variants only after multiple passages of undiluted virus on fresh cells. Restriction analysis showed that the td virus produced by Q3B cells is composed of two types of genomes: one lacking the entire v-src gene and the other carrying partial deletions of this gene predominantly located in the amino-terminal portion of the coding region of v-src. To study the mechanisms of these partial deletions, we molecularly cloned and sequenced the v-src genes of several td proviruses. We show that these mutants carry single or multiple v-src deletions of limited size, presumably generated by multiple mechanisms. Two deletions of 170 and 112 bp located in the 5' portion of v-src are frequently generated during RSV replication in Q3B cells and may represent preferential sites for v-src deletion in these cells.