The intracellular segments of classic adhesion molecules such as N-CAM
do not show structural similarity to any known signaling molecules. T
his suggests that their effects on signaling responses must be exerted
indirectly through associated proteins. In contrast, many receptor pr
otein tyrosine phosphatases (RPTPs) possess extracellular segments wit
h homology to cell adhesion molecules linked directly to intracellular
segments comprising one or two protein tyrosine phosphatase catalytic
domains. Therefore, the RPTPs have the potential for direct modulatio
n of catalytic function through engagement of the extracellular segmen
t, suggesting they could be direct signal transducers of cell contact
phenomena. in the past few years, some RPTPs have been shown to effect
cell-cell adhesion directly via hemophilic binding or indirectly by a
ssociation with known cell adhesion molecules. In addition, RPTPs have
been localized to points of cell-cell or cell-matrix contact, indicat
ing their potential to regulate these structures.