Protein tyrosine kinases (PTKs) regulate a wide variety of cellular responses, including responses to mitogenic signals from growth factors, activation signals that trigger specialized cellular responses (eg secretory responses in neuroendocrine cells, and platelet aggregation), adhesion signals promoted by the extracellular matrix, and signals that stimulate cell motility. A growing body of data indicates that the Src family of PTKs functionally interacts with a variety of receptor and nonreceptor PTKs, diversifying the signaling pathways mediated by such PTKs [1]. The cellular Src protein (hereafter referred to as Src) is the prototype of this large family of related kinases (numbering nine at the most recent count), all of which share a common domain structure. All members of the family are acylated at the amino terminus (some members are acylated at more than one site), and all contain a'unique'domain (which distinguishes them from other members of the family), $ rc homology (SH) 2 and 3 domains, a highly conserved catalytic domain, and a short carboxy-terminal tail sequence [2]. Most members of the Src family are highly regulated, exhibiting little activity in normal cells in the absence of an activating signal. This tight regulation is mediated by phosphorylation of a tyrosine residue in the carboxy-terminal tail, which in turn interacts in an intramolecular fashion with its cognate SH2 domain. In this conformation, Src appears to be enzymatically inactive and unable to mediate protein-protein interactions via its SH3 and SH2 domains [2].

The purpose of this review is to summarize recent data that point to the role of Src (and several members of the Src family, most notably Fyn and Yes) in cooperating with signaling responses emanating from PTKs that are activated by growth factors and components of the extracellular matrix (Fig. 1). Here, we discuss the possible roles of Src (and, in some cases, Fyn and Yes) in modulating signals that control both growth and cell movement.