The Crk proteins, originally isolated as an oncogene product in an avian sarcoma virus (26), belong to a class of adapter proteins containing Src homology 2 (SH2) and SH3 domains. Other members of this group include Grb2/ASH (11) and Nck (13). These bifunctional proteins are thought to couple tyrosine-phosphorylated receptors or their substrates via the SH2 domain to downstream effectors via the SH3 domain. Three cellular homologs of v-Crk have been identified: Crk-I, a 21-kDa protein with only one SH2 and one SH3 domain (24); Crk-II, 40-and 42-kDa proteins consisting of one SH2 and two SH3 domains (36); and CrkL, a 36-kDa Crk-like protein with one SH2 and two SH3 domains (49). Despite their lack of demonstrated enzymatic activity, expression of v-Crk or Crk-I but not Crk-II leads to cell transformation and increased tyrosine phosphorylation (24,27). Furthermore, v-Crk binds directly to the major tyrosine-phosphorylated proteins present in v-Crk-transformed cells, presumably via its SH2 domain (22, 23). These phosphoproteins include paxillin, a focal adhesionassociated protein (3), and the newly identified p130 cas (39). p130cas has no known catalytic domain but contains a cluster of predicted high-affinity Crk-SH2 binding motifs and forms a stable complex in vivo with v-Crk. The Crk-SH3 domain has been shown to interact with several proteins, including Abl (8), Sos (9, 21), C3G (9, 15, 21, 46), and, more recently, Eps15 (42). Since Sos and C3G contain a guanine nucleotide exchange activity, Crk proteins have been hypothesized to play a role in the regulation of p21 ras -GTP formation. Both the SH2 and SH3 domains of v-Crk are required for the transformation of chicken embryo fibroblasts (27). In addition, Crk mutants with dysfunctional SH2 or SH3 domains inhibited neuronal differentiation of PC-12 cells and nerve growth factor-induced activation of Ras (21, 45).The c-Abl oncogene product is a member of the nonreceptor class of tyrosine kinases. Like c-Crk-II, c-Abl overexpression does not lead to cellular transformation (10,14). However, the chromosomal translocation of c-abl to the bcr gene in Philadelphia chromosome-positive human leukemias produced the 210-and 190-kDa chimeric Bcr-Abl proteins (6, 16). These fusion proteins have elevated tyrosine kinase activity and transforming properties (20,32). Although the precise mechanisms involved in cellular transformation by Bcr-Abl remain unclear, several key signaling proteins are known to be tyrosine phosphorylated in cells bearing this oncogene, including Bcr-Abl itself (32) as well as Shc (35), CrkL (48), paxillin (40), and the proto-oncogene product of cbl (1). Moreover, activation of Ras has also been implicated in Bcr-Abl-mediated cellular transformation (34,35).To evaluate the potential role of Crk proteins in the transforming functions of Bcr-Abl, we have examined their interactions in the human chronic myelogenous leukemia cell line K562. We report here that, unlike observations in v-Crk-and v-Src-transformed cells, p130cas is not tyrosine phosphorylated...